America's Highways 1776–1976: A History of the Federal-Aid Program/Part 2/Chapter 6
U.S. 422 in rural Pennsylvania. The curvilinear alinement and roadside treatment have resulted in a design incorporating utility, beauty, and economy of maintenance.
Highway design is the art of anticipating the roadway requirements of motor vehicle traffic and of satisfying these requirements in the preparation of construction plans. It involves the selection of dimensional values for geometric features such as widths, radii of curvature, rates of cross section slopes and longitudinal gradients; the determination of the requirements for the roadway structure (pavement and base courses) and for the bridges, drainage and other structures ; the preservation and restoration of ground cover and plant growths; the safety and efficiency of traffic operations, including all forms of traffic control devices; and the melding of the highway’s alinement and gradient with the landscape in a manner least disturbing to the natural environment. Engineering analyses during the highway design must attain an acceptable balance between the broad controls of economic limitations, land space usage and environmental and social considerations.
The roads existing during the early years of the automobile were not designed for motor vehicle usage; they were simply wagon roads. For many years after the automobile came into wide use, these same roads were adapted to automobile travel with little or no change in location, alinement or geometries. Even after the automobile became commonplace, a number of years, or even decades, were to elapse before highways were constructed to meet the unique requirements of motor vehicle traffic.
The transitional period from horsedrawn to motor vehicle traffic occurred at a time when railroad transportation was approaching maturity. Trains shared with the automobile one distinguishing characteristic when compared with horsedrawn vehicles—both traveled at a faster speed. It was natural, then, that when travel speed became recognizable as a factor in highway design, the leaders in this art turned to the practicing railroad designers for guidance in determining alinement design details to satisfy the requirements of the higher speeds of motor vehicles.
While building upon these bases of knowledge, highway designers were often in a dilemma as to which features of wagon roads should be retained, modified, or rejected, which characteristics of railroad practice should be incorporated, and what new concepts were needed to satisfy the peculiar qualities of automobile traffic. Resolution of these matters was often the subject of heated debate with the answers frequently being dictated by economics.
From the standpoint of highway design, the automobile era logically falls into four fairly distinct periods: (1) 1900–1920: the preemptory period during which the motor vehicle usurped the wagon road in its contemporary state of improvement; (2) 1920–1930: the uplifting period during which existing roads were adapted to motor vehicular use, largely by shaping the cross section, draining and surfacing; (3) 1930–1940: the period of stabilization in the design of conventional highway during which the dynamics of motor vehicle traffic were recognized as a major force to be dealt with; and (4) 1940–1976: the era of the freeway.
It is significant that throughout the brief period of development of the modern highway system there has been a lag of a decade or more between the time of recognition of a new design principle, or a standardization of a dimensional value, or the advocacy of a concept, and the time that results of these advances in design begin to show in completed highways. For example, a discovery or determination in 1910 that highway curves should be superelevated to offset centrifugal force found little application until after 1920 when the extensive program of surfacing highways came into full swing.
The categorization of the four design periods reflects the period of application of design principles, rather than the period of their discovery. Each of these periods is discussed in terms of the design characteristics in vogue at the time and of the factors that influenced these characteristics.
Twilight of the Wagon Road: 1900–1920
The automobile came on the scene at a time when roadbuilding was undergoing a renaissance after 50 years of neglect. The better rural roads of the mid-19th century were waterbound macadam, 12 to 15 feet wide—a width that was adequate for two loaded wagons to pass each other with the horses at a walk. To. shed water quickly, these roads were crowned on both the straight sections and the curves, and the crown, 6 to 8 inches higher in the center, was steep enough to make driving at the sides uncomfortable. Consequently all traffic ran in the middle, taking to the righthand side of the road only to pass other vehicles.
In 1913, even mountain roads such as this earth road in Wise County, Va., had relatively flat grades to accommodate horsedrawn vehicles.
Wagon roads outside of cities were usually unsurfaced except for the more heavily traveled trunk routes. Highways carrying heavy tonnages of freight were surfaced with broken stone, gravel, or water-bound macadam. The National Road, for example, which was constructed westward from Cumberland, Maryland, in the early years of the 19th century, had a 20-foot surface consisting of a 12-inch bottom course and a 6-inch top course of broken stone. This was far superior to the typical highway of that period.
In addition to macadam, broken stone, and gravel, other road surface materials used with varying success were brick, asphalt, wooden planks, sand-clay and granite blocks.
The basic principles for constructing all forms of surfaces as they are known today had been discovered and put into limited practice before 1910. Except for streets in the larger metropolitan areas and major routes of commerce, most road surfaces were in a rather primitive state of improvement at that time and were to remain so for several years to come.
These roads were built for horsedrawn steel-tired traffic traveling at a top speed of 8 miles per hour. In recognition of the limited capability of animal-drawn conveyances to ascend grades, gradients seldom exceeded 5 percent, and this resulted in rather crooked locations carefully selected to avoid steep grades, closely fitted to the terrain, with small cuts and fills to save grading costs. Side slopes in both cuts and fills were as steep as the natural materials would allow, usually 1½ feet horizontal to 1 foot vertical.
In the era of animal power, 105 feet was a generous radius for horizontal curves. This would enable a four-horse team and wagon having a total length of 50 feet to round a curve without leaving a 12-foot travelled way. For two-horse rigs with wider roads, the radius could be even shorter. Vertical curves were seldom used; the vertical angle between two 5 percent grades was only about 6 degrees, and this was hardly enough of a peak or valley to cause discomfort to anyone in a vehicle traveling at 4 or 5 miles per hour.
The wagon roads were inconspicuous and, from economic necessity, “rested lightly on the land.” However, it is doubtful that their builders considered this fact as an esthetic advantage for their work or, for that matter, that they gave much thought to the appearance of the road itself. In a few eastern States, notably Massachusetts and Maryland, trees were planted along the roads, but these had a practical purpose in addition to creating visual beauty. Their shade reduced the dusting and disintegration which followed when waterbound macadam surfaces dried too rapidly.
Early Location Methods
The railroads pioneered good location methods in the early 20th century. Under the spur of competition among themselves and being relatively unhampered by manmade obstacles, the railroads began to seek locations with flatter gradients than previously accepted in the interest of greater loads and less fuel consumption. For such exacting requirements, the old method of location by which the locator went into the field, selected the route by direct observation, and set the stakes as he went along was no longer adequate in any but the most level country. The locator could be aware of only what was in the range of his vision; a better location might lie over the next hill, but he could not see it. A method had to be devised that would permit the locator to examine a wider and longer sweep of country, and to meet this need, a new method of location evolved. This is called the “topographic method” to distinguish it from the old “direct method.”
Instead of staking the centerline directly on the ground, the locator surveyed a preliminary line, or “baseline,” as a base for a strip topographic map. After completing 4 or 5 miles of topography, the locator spread his maps on a long table, and he looked down upon them as if he were an observer in a balloon. Instead of seeing only a hill in front of him he saw the country in a miniature far ahead. His vision was not obstructed by trees, and he was not annoyed by insects or the weather. He easily perceived the vital features, or “controls,” which determined the best location for the centerline that would conform with the railroad’s geometric standards.
By scaling the horizontal distances between points where his projected centerline intersected the contours of the map, the locator obtained a profile which he could analyze for gradients and earthwork balance. If this profile was unsatisfactory, he could draw another paper projection, scale a new profile, and make a new analysis. Only when he was satisfied that he had the best location did he transfer the paper projection to the ground as his final staked location for the centerline of the railroad.
It was to be many years before the art of highway location and design was to attain such a high degree of sophistication. Until about 1930, the railroad method of alinement and profile was generally considered the ultimate in design for highways as well.
The Impact of the Automobile on the Early Roads
Prior to and during the first decade of the 20th century, the automobile had a negligible effect on the design and construction of highways. It was looked upon as an interloper which had to adapt itself to existing streets and highways. If adjustment was necessary, it was encumbent upon the vehicle manufacturer and the operator to make such adjustment. It was considered no more inconvenient for a motorist on a rural highway to slow to 6 or 8 miles per hour for a right-angled curve than for him to do so at an urban street intersection. If climatic conditions were unfavorable for motoring, the traveler merely substituted the horse for the auto. In northern latitudes, roads were so frequently impassable for automobiles during the winter months that cars were customarily placed on jacks in the late autumn and remained there until after the spring thaws in order to preserve the tires.
Despite enactment of the 1916 Federal Aid Road Act, the harsh impact of the automobile on the highway system was felt very strongly in the second decade of the century, and this was particularly true of the roadway surface. The automobile was extremely damaging to macadam and gravel roads, which essentially were held together by the interlocking of stone 
TYPICAL SECTIONS 1900–1920
When roads became impassable, some motorists fell back on the services of more traditional transportation.
Another unexpected effect was greatly increased wear on the inside of curves and the loosening and shifting of surfacing materials from the inner to the outer side of the roadway on curves. Rather than slowing to a speed commensurate with the radius of the curve, drivers instinctively hugged the inside of the curve, even to the extent of encroaching upon the ditch slope, in order to lengthen the effective radius and to take advantage of the “banking” afforded by the ditch slope.
In the case of curves to the left, this meant crossing into the opposing lane, since the normal, high crown was customarily carried around curves. As a consequence of resistance to lateral acceleration, loose surfacing materials were thrown to the outer side of the curves, resulting in a curve of longer radius that became somewhat superelevated by traffic, rather than by design and construction.
The impact of the automobile on the early roads was not all on the negative side. In the case of gradient, for example, automobiles and trucks were capable of ascending any grade suitable for horsedrawn traffic. If the surface provided sufficient traction, grades up to 6 or 8 percent posed no serious problem for skilled operators who had mastered the art of down-shifting to a lower gear. One feature of vehicular design that was the cause of considerable consternation to many drivers ascending hills was the location of the gas tank with respect to the carburetor. Many vehicles, including the most popular make of the era, relied upon gravity flow of fuel from a tank beneath the driver’s seat to the engine’s carburetor located only 2 or 3 inches lower than the bottom of the tank. It was not uncommon to stall on a grade for lack of gas, even with a tank partly filled. There is no evidence that these experiences altered the design of highways, however.
By the year 1920, many State highway departments were recognizing the inadequacies of the design features of wagon roads, but aside from a very limited mileage of added pavement during the period 1900–1920, advances in design were very limited. It is questionable whether rural roads in many of the States up to this time were actually designed. Rather, they came into being through the gradual improvement of trails, and their shape and dimensions depended more on the skill and judgment of a maintenance man riding a split log drag than upon a professional design engineer guided by a set of design standards.
By the early 1900’s the National Road, a superior highway for its time, needed improvement. This is a section of the road near Hancock, Md., before construction.
In a report prepared by the highway commissioner of one of the States in 1915, county officials were advised:
At about the same time, a neighboring State reported that 90 percent of the roads within its boundaries were earth roads and added, prophetically, that they were likely to remain earth roads for a great many years.[2] In 1933, 18 years later, less than 14 percent of the total road mileage in that State was reported as surfaced.[3] Despite the fact that over 13,000 miles had been paved in the interim, there had been little gain percentagewise because of expansion in the highway network. Many of the counties . . . have the remains of roads . . . built during the early part of the nineteenth century . . . The old roads of this age still stand as monuments to the ability and farsightedness of our forefathers . . . The roads of that day were arteries of commerce. Later they gave way to the railroads and roadbuilding became a forgotten and lost science. Any man who could nap rock was a roadbuilder.
With the coming of the automobile and the motor truck the road is again the artery of commerce. Interest has been revived. We are beginning to appreciate the State highways and turnpikes built nearly one hundred years ago. We find them hard to improve on, even with present day methods. We can build a better surface, but we can’t build a better subgrade. . . .
. . . a road once built is rarely changed, provided it is built according to certain laws which are as old as the hills themselves. So insist that your road be properly drained, well located, taking advantage of every topographic condition. No grade should exceed a rise of 5′ in 100′, and lastly that you keep and take sufficient right of way to allow for ditches and any increase in width your road may require in the years to come . . . The right of way, roadbed and drainage openings are permanent. The surface, regardless of what it is, will sooner or later have to be replaced.[1]
The same section of road after construction.
Nationwide, the mileage of high type pavements on rural highways in 1920 was negligible in relation to the total miles of highway, although substantial gains had been made in gravel surfacing up to that time. Regretably, however, much of the rural pavement in place at the beginning of World War I was destroyed by heavy trucking in pursuance of the war effort.
Dawn of the Motor Highway: 1920–1930
The period 1920–1930 may be characterized as a period of trial and error in adapting an assortment of roads laid out for use by horses and wagons to the use of automobiles and trucks. While the highway system continued to expand during this period, particularly in the agricultural belt of the Plains States, the nature of the added highways was generally the same as the earlier wagon roads with perhaps the addition of an all-weather surface, frequently gravel or sand-clay. Except for roads in the New England and North Atlantic States and in portions of the Pacific States, few rural roads in 1920 were surfaced with anything better than sand-clay or gravel. If one were careful in selecting his route, he could travel from New York City to Washington, D.C., on paved highways. From Washington to Richmond, however, the best route was one-third longer than the most direct one and included 99 miles of gravel surface rather than a paved surface. From Chicago to Milwaukee, according to a prominent guidebook of the day, the entire 92 miles was paved with concrete or macadam “except for one very rough mile.” Going south from Chicago, the route to Lafayette, Indiana, consisted of 98 miles of gravel, 26 miles of macadam, and 16 miles of dirt. The entire 102-mile route from Atlanta to Macon, Georgia, consisted of sand-clay roads.[4] With these segments of interstate highways in the conditions described, it requires little imagination to visualize the condition of the lesser important routes. Clearly, the most pressing need was for all-weather surfacing, and this was the course that the roadbuilding program followed.
Geometric Design Features
While most highway authorities were conscious of a need to modify the highway cross section and alinement to meet the requirements of motorized traffic, there was little precedent upon which to base judgments of immediate design requirements, or worse, for predicting those of the future. Highway engineers had much in common with one self-made automotive engineer of this period who has been quoted to the effect that, “If its in a book its out of date.”[5][N 1]
Elements recognized as warranting special consideration because of higher vehicular speed included sight distance, curvature, and superelevation. Variables taken into account because of the volume and character of traffic were pavement width and structural needs. The manner of satisfying these requirements varied not only between States, but within States. Minnesota, for example, found that, “Abrupt narrow curves have contributed their share to fatal accidents . . . which it is expected will be relieved by . . . [a] requirement for a clear sight distance of 200 feet on all State roads with widening and banking on curves.”[6] In 1921 it was the practice in Illinois, as another example, to eliminate right-angled turns “whenever possible,” and where such turns were necessary, a minimum radius of 500 feet was used. Moreover, “All curves which have a radius of less than 6,000 feet are super-elevated . . .”[7] The speed upon which the superelevation was based was the legal speed limit which, in 1921, was 25 m.p.h. This was increased to 35 m.p.h. in 1928, with a maximum superelevation of 1 inch per foot for curves with radii of less than 1,000 feet.[8] The State of California, among others, had earlier considered the matter of superelevation and, in 1917, had reached a compromise decision to apply superelevation to concrete pavements on mountain roads only. The rate of superelevation to be applied varied from a maximum of ¾ inch per foot for curves having a radius of 75 feet or less to ⅛ inch per foot for radii within the range of 225 to 300 feet.[9] A few States used spiral transitions, but the practice was not widespread. Obviously, most decisions in the matter of roadway geometry were arbitrarily made and had little scientific support.
- ↑ The engineer was Childe Harold Wills, at the time an employee of the Ford Motor Company and later producer of the Wills St. Claire Automobile.
Perhaps the element that varied most widely was the width of surfacing. In Kentucky, for example, during the years just before and after 1920, contracts were awarded for pavements ranging from 9 to 20 feet in width, with 14 feet predominating.[10] Most bridges had either 12- or 16-foot roadways. In Illinois during the same period, concrete and brick pavements were 10, 15, and 18 feet wide.[11] It was common practice at about this time to construct pavements with sufficient width to accommodate only a single lane of traffic. There was some disagreement as to whether the best position for such a pavement was in the center of the roadbed or at the right-hand side going to the market center. One critic claimed that the center was the most desirable position because it was better looking, was more easily drained, and was safer to traffic.[12]
The dilemma as to the most suitable position for single-lane pavements was soon resolved—the rate of traffic growth was so rapid that single-lane pavements were short lived and such construction was uneconomical. Consequently, this type construction was discontinued. The Federal Highway Act of 1921 doubtless spurred this action. This legislation required, for the first time, that the wearing surfaces on Federal-aid highways be at least 18 feet wide.
With the discontinuance of single-lane construction, practically all subsequent paving between 1920 and 1930 was of two-lane width, ranging from 14 to 20 feet. Pavements wider than two lanes were rarely needed. The experience in Maryland may be typical. In 1920 the concrete roads were constructed 15 feet wide, 6 inches in depth at the edges and 8 inches in depth at the center, resulting in a crown of 2 inches. This same type construction was used in 1921. In 1922 the Commission thought it desirable to reduce the crown and adopted a section 6½ inches in depth at the edges and 7½ inches in the center. This construction method was used in 1922 and 1923. At that time, however, concrete was finished by hand, and some difficulty was experienced in getting a smooth road with only a 1-inch crown. In 1924 the Commission went back to building a 2-inch crown, still using a 15-foot width, 6 inches in depth at the edges and 8 inches at the center. This type construction continued to the end of 1927. In 1928 the thickened edge was adopted, and the standard width of roadway was increased to 16 feet with a depth of 9 inches at the edges and 6.3 inches at the center and was the method still used in 1930.[13]
In contrast, Illinois during this period was building concrete pavements 18 feet wide (in some cases 20 feet) on a roadway 30 feet wide. In 1926 Illinois constructed its first four-lane highway.[14] Lanes 10 feet or more in width were rare anywhere in the country.
Horsedrawn equipment making a cut and raising levee across Souwashy Creek Bottom near Meridian, Miss.
Except where the terrain was flat, the rolling profile and widening alinement of the wagon road era continued. Although location and design engineers recognized the desirability of gentle curvature and uniform grades, much of the design was dictated by economics and the limitations of grading equipment. Grading was performed largely with animal-drawn equipment; hence heavy cuts and fills and direct alinement were avoided in favor of sidehill locations. Power shovels were in common use for rock excavation and sidehill casting, but the narrow cuts and fills of the period were too confining for efficient use of motorized hauling units, which were also poorly adapted to operating on newly placed embankments. For the year 1920, Minnesota reported that equipment actually engaged in highway construction included 2,693 scrapers (wheel, slip, or Fresno), 36 steam shovels and 6,212 horses and mules, together with 466 motor trucks recently acquired as war surplus materials.[15] In Illinois in 1924, a recordbreaking year for construction in that State, 11,700 men and 3,000 teams were employed, a “team” consisting of two or more animals. The day of mechanization was still in the future.
Roadbuilders then, as always, were realists. They were in a race with time to pave as many miles as possible with available funds. Office seekers made extravagant promises for getting the voters out of the mud. The report of one highway department in 1924 boasts that of the 4,671 miles of hard surfaced roads then existent in the State, 75 percent were completed during the 4-year tenure of the encumbent governor.[16] One result of this frenzied effort to add miles of pavement was that many design features, later found to be hazardous, were incorporated into the highway system before a background of experience could be developed.
Every section of newly paved highway seemed to have its “deadman’s curve.” Vehicular brakes were generally poor, as was lighting equipment. Many trucks using the highways in the late twenties were still equipped with acetylene or kerosene lamps, and their speeds were limited by mechanical engine governors to 15 to 18 m.p.h. A good number of horsedrawn vehicles continued to use the public highways. Passenger car speeds increased rapidly in keeping with highway and automotive improvements. In 1920 most States had a 25 m.p.h. speed limit for rural highways. By 1930 the speed limit most frequently encountered was 35, with a few States having limits of 40 or 45 m.p.h. No highway, regardless of how designed, could be properly fitted to the needs of such a conglomeration of vehicles and regulations. Not surprisingly, deaths and injuries soared.
There were also signs of pessimism in some quarters as to the ability to cope with the unbridled growth in motor vehicle usage through highway construction alone. Taking note of the heavy weekend demands being placed on improved highways, one State highway commission noted in 1926:
It seems quite clear to the commission that the regulation of traffic is the first step toward adequate use of the highways. Indeed, the number of vehicles placed upon the highways each year is proportionately greater than the miles of highway which are added. The conditions of traveling, instead of becoming better, are becoming worse, due to congestion on these roads, and the entire solution does not seem to lie in the building of more roads, but rather in the regulation of traffic that goes over them.[17]
This philosophy was to find expression many times over in the years ahead. Traffic, however, continued to increase in volume.
Partly as a result of the wide disparity in design practices between the States, a new form of leadership in the art of highway design began to assert itself toward the end of this period. This was the Committee on Standards of the American Association of State Highway Officials (AASHO), which had been formed in 1914. Initially, AASHO’s Committee on Standards confined itself to disseminating information on design to its members, but in 1928 it proposed that the Association adopt “standards of practice” to guide the member States in technical matters in which some uniformity from State to State was urgently needed. The resulting first standards, 1928, prescribed:
- That whenever practicable, shoulders shall have a standard width of not less than 8 feet.
- That on pavements, 10 feet shall be considered as the standard width for each traffic lane.
- That the crown of a two-lane concrete pavement shall be 1 inch.
- That no part of a concrete pavement shall have a thickness of less than 6 inches.[18]
The AASHO Committee on Standards and its offspring, the Committee on Planning and Design Policies, were destined to have a profound effect in stabilizing highway design by recognizing and advocating the best and most economical practices of the various highway departments and in promoting the results of research efforts that were soon to be undertaken.
Advent of Aerial Surveying
Another development of the decade, 1920–1930, was the initial use of aerial surveys in highway location. Mention has been made of the “topographic method” pioneered by the railroads for selecting the most suitable alinement from among a number of alternatives. Initially the topographic method did not meet with much favor among highway builders for several reasons: (1) Alinement and profile were not as critical for highways as for railroads; (2) the method was slower and more costly than the conventional or direct method; and (3) most highway projects consisted of improving an existing or established route, which might be either a trail, a well traveled artery, or something in between. The penalty for using the direct method was the production of an inferior, unsafe highway.
When aerial photography became available on a commercial scale, it offered a means of overcoming the greatest objection to the topographic method of location, namely, heavy expense in time and manpower in making field surveys and preparing maps. The highway profession was quick to recognize the potential of aerial survey methods. An example of its use described briefly in the 1927 Aircraft Yearbook:
The Bureau of Public Roads of the Department of Agriculture has used the airplane in making of mosaics to show the route of a proposed road and to ascertain over what territory the proposed road will travel. The Army Air Corps has provided the equipment and personnel, and the Bureau has provided the films and paid the cost of operation.
During the past year aerial survey has been made of a proposed new highway from Washington to Mt. Vernon, Va. The survey showed the old road as well as the territory over which the new road would have to be laid. Surveys have been made in Connecticut along the coast line, and again between Boston and New York in the vicinity of the Boston Post Road, in an endeavor to find a new automobile route to take care of the crowded travel on the old Boston Post Road.[19]
The article concluded that the saving in time and money on these and other projects was very satisfactory, and future use of this means of mapping and surveying was contemplated. This was a very conservative prediction.
A typical main highway of the early 1920’s. The major defects are the 16-foot concrete pavement width, narrow and poorly maintained shoulders, and the encroachment of utility poles.
Stabilization in Design Practices: 1930–1940
The period 1930—1940 was one of discovery, as well as of stabilization. On the debit side, administrators were dismayed to discover that many highways paved during the previous decade, in the expectation they would last a lifetime, were already obsolete. There were several reasons for this—the alinement was not suited to current speeds, sight distances were too short, pavements and shoulders were too narrow, and traffic volume had simply outgrown the capacities of many two-lane roads. Moreover, the seemingly unlimited capacity of multilane roads near cities was being rapidly depleted as a result of uncontrolled access. Pavements were deteriorating because of the frequency of heavy axle loads. Trucks were seriously interfering with the free movement of passenger cars where there were long steep grades.
On the plus side, the most outstanding discovery, or more properly, achievement, was the recognition that most problems could be resolved through a co-ordinated research effort coupled with an organized program to exchange knowledge and information gained in the various States.
A coordinated research program was given a strong boost by the U.S. Congress in 1934 through passage of the Hayden-Cartwright Act. This Act authorized the use of up to 1½ percent of Federal-aid highway funds for planning and research.
Secondly, distilling the results of experience and research and promoting the best in design practices commensurate with economic benefits found expression through the AASHO Committee on Planning and Design Policies.
Design Policies
In February 1937 a proposal was approved by AASHO to establish a special committee consisting of three key officials from the Bureau of Public Roads (BPR) and 12 outstanding design engineers from the States. This committee was named the Committee on Administrative Design Policies. It was subsequently renamed the Committee on Planning and Design Policies, and membership was increased to include 20 members from the States with BPR furnishing a single member, a nonvoting secretary. A unique feature of the committee working process was the provision of a small force of experts assigned by BPR to devote full time, if necessary, to the work of the Committee.
The Committee’s mode of operation was to outline a general program of work, after which the BPR task force gathered and evaluated all the known information on each subject. If there were gaps in the existing knowledge, the BPR engineers identified them for further study. Eventually the staff prepared a tentative discussion, with indicated design controls, guide values and other conclusions for that subject. This was then criticized, evaluated, and supplemented by the Committee members and reworked until a policy acceptable to them was produced. The resulting policy was submitted through the Committee on Standards to the AASHO Executive Committee for ballot by the several States; with a two-thirds favorable vote, it became an approved policy, and also, in effect, the national design policy of the United States on that particular subject.
The Committee soon developed design policy brochures on seven projects: (1) Highway classifications; (2) sight distance; (3) marking and signing no-passing zones; (4) highway types; (5) intersections at grade; (6) rotary intersections; and (7) grade separations. These booklets have considerable historical significance since, together, they are the fundamental structure upon which all subsequent geometric design policy for highways has been based.
A Policy on Highway Classification, 1938—offered a method of classifying highways to indicate the service expected of them. Three factors were considered in the classification: (1) Traffic volume, (2) character of traffic, and (3) design speed.
“Traffic volume” represented the number of vehicles per hour and was defined as the average of the probable maximum hourly traffic of several peak days. Prior to this time, it had been common practice to consider traffic on a daily basis if, in fact, it was considered at all in geometric design. In later years, research was to permit a degree of refinement, and the 30th highest hourly volume of the design year became the criterion of traffic volume classification for design purposes.
“Character of traffic” was used to denote the relative number, or percentage, of trucks and buses in the traffic stream in order that allowance could be made for them in design and operation of highways. Three categories were utilized: “P” for traffic composed entirely of passenger cars or types of trucks which did not impede smooth traffic operation; “T” for traffic in which the percentage of trucks likely to use the highway was such that movements of passenger cars would be interfered with and, consequently, should be given detailed considerations; and “M” for mixed traffic where the percentage of trucks was between that for “T” and “P.” While the classification was necessarily vague, it did serve a useful purpose in recognizing that truck traffic was an essential factor that must be dealt with. This classification stood for many years and was discontinued only after a means was devised in 1950 for converting truck volumes into equivalent passenger car volumes.
The “assumed design speed” was used for correlating the design features of a length of highway that affect, or are affected by, the speed of operation. For this purpose, assumed design speed was chosen as being representative of the maximum approximately uniform speed that probably would be adopted by the faster group of drivers, exclusive of the reckless few, that would use the highway. Speed classifications of 30, 40, 50, 60 and 70 miles per hour were agreed upon, and guidance was offered as to conditions under which each might be appropriate.
Adoption of speed as a design criterion was in recognition of the fallacy of the previously held belief that drivers could be relied upon to detect sharp curvature and short sight distances sufficiently well to adjust their speed to conditions. The false assumption that drivers would reduce their speed to as low as 15 or 20 miles per hour at curves was a major cause for high accident rates and the early obsolescense of many highways paved prior to 1930. The design speed concept was truly a landmark innovation in highway engineering.[20] 
Center markings on curves aided motorists on this highway in 1921, but short sight distances made passing dangerous.
A Policy on Sight Distance for Highways, 1940—provided a scientific approach to answering the controversial question as to the length of sight distance that should be provided to assure safety at curves and crests and also for overtaking and passing slower vehicles on two- and three-lane highways.
In the absence of research results, which were to become available a few years later, certain assumptions were necessary in deriving the values. It is a credit to the Committee that the assumptions were remarkably accurate. In one of the first applications of the design speed concept, the derived minimum sight distance values for stopping ranged from 200 feet for an assumed design speed of 30 m.p.h. to 600 feet for a design speed of 70 m.p.h. It was further postulated that this distance should be measured on a line from the driver’s eye 4.5 feet above the surface to a hypothetical object 4 inches above the pavement.[21] There was considerable conjecture as to the height of the object. Logic would dictate that visibility of the road surface itself would provide the ultimate in safety, but provision of the required sight distance to the road surface would necessitate extremely long vertical curves at hill crests and would, therefore, be very costly. A standing or slow moving vehicle would be the type of obstacle most likely to be encountered on a highway and a height of 2 feet, representative of the height of tail lights for a typical vehicle, was favored by some Committee members. Obstacles a foot or so in height, even though encountered only infrequently on highways, could result in serious accidents if struck by cars and, consequently, such minimum heights were rejected on the grounds of being unsafe. The 4-inch dimension finally agreed upon represented a compromise between economics of construction and severity of hazard. (Both dimensions, height of eye and height of object, were reevaluated in 1961 and, because of reduced height of vehicle, were changed to 3.75 feet for height of eye and 6 inches as a compromise value for height of object.)
Passing sight distances were derived by dividing the passing maneuvers into several component parts and developing time-space relationships for each. Desirable distances, as well as absolute minimums, were developed. For two-lane roads, the “desirable” values ranged from 600 feet for a design speed of 30 m.p.h. to 3,200 feet for a speed of 70 m.p.h. The sight line was from the driver’s eye, assumed to be 4.5 feet above the road surface, to the top of an on-coming car, also assumed to be 4.5 feet in height.[22] Both dimensions were changed to 3.75 feet in 1961 in recognition of changes in vehicle design.
The three-lane highway, like the single-lane highway of 20 years earlier, enjoyed a comparatively short period of popularity, and sight distances for such highways, as well as for two-lane highways, were discussed in this policy. Very few three-lane projects were constructed before 1930 or after 1940. It was soon learned that, while they did provide greater capacity than two-lane roads, the added increment was not great. With the rapid increase in traffic, they were soon taxed to capacity. Moreover, they did not lend themselves to conversion to multilane divided highways. Of greatest concern, however, was their poor accident experience. One reason for this was that sight distances were frequently too short for passing. The AASHO policy sought to overcome this deficiency in future construction by furnishing derived values for needed passing sight distances on three-lane roads. “Desirable minimum” values ranged from 900 feet for 50 m.p.h. to 1,700 feet for 70 m.p.h. Another reason for the poor accident experience on three-lane roads was the lack of uniformity and the haphazard manner in which they were marked to regulate overtaking and passing during the early years of their use. A recognized need for improvement in this department led to development of the next of the several policies.
Kentucky’s Daniel Boone Parkway exhibits modern design features such as sweeping curves with adequate sight distances and gentle grades.
A Policy on Highway Types (Geometric), 1940—outlined the distinctions between the two-, three-, and four-lane highway types and divided highways. It dealt primarily with pavement widths and factors of driver behavior and highway design that affect width. The policy related the traffic volume, the character of traffic (“P,” “M” or “T”), and design speed to the minimum width of pavement that should be provided. For two-lane roads, the minimum width of surfacing varied from 16 feet for the lowest classification upward to 24 feet. (Single-lane roads were acceptable for volumes below five vehicles per hour.) Turning paths were developed for three design vehicles—a passenger car, a truck, and a tractor-semitrailer.
The Committee lamented the absence of better highway capacity information that would permit delineation between the traffic warrants for two-, three-, and four-lane pavements. Because of the intervention of World War II and for other reasons, such information would not become available for another 10 years, although the basic research was already well advanced.
The policy discussed median design for divided highways at considerable length, as well as curbs, sidewalks, guardrails and shoulders. Shoulders 8 to 10 feet wide, clear of all obstructions, were recommended.[23]
A Policy on Intersections at Grade, 1940—treated the subject in great detail, utilizing turning paths for a design passenger car and a design truck. Design requirements were developed for various types of intersections from a simple crossing to the more elaborate types of channelized intersections with relatively high-speed turning roadways and speed change lanes. Innovations included three-centered compound curves for the pavement edge at turns, minimum radii for separate turning roadways as related to design speed, pavement widths for such roadways, and sight distance requirements at intersections not controlled by signals. Vehicle dimensions have changed since this policy was prepared, but in all other respects the concepts have stood the test of time with little need for modification.
A Policy on Rotary Intersections was not completed until 1941. During the period 1930–1940, rotary intersections were thought to be a considerable improvement over conventional intersections, so much so that they were often constructed as substitutes for grade-separated interchanges. Events were to prove that, like the three-lane road, their effective life was usually rather short because of their limited capacity and the rapid rate of traffic growth.
The policy provided guidance in selecting various design dimensions such as radius of the central island, roadway widths, and lengths of weaving sections as related to design speed.
A Policy on Grade Separations for Intersecting Highways was the logical sequel to the two policies on intersections. It was the final one of the seven geometric design policies, and it was completed in 1944.
The title of this policy is somewhat misleading because it treated not only the separating structure, but actually covered in great detail the design of grade separated traffic interchanges. Design data were grouped in three categories: (1) Structures and approaches, (2) ramp arrangements, and (3) ramp design.
One important design element not treated in detail by these several design policies was the relation between curvature, superelevation, and design speed.
Curves
By 1930 most States were superelevating all curves except those of very long radius; however, there was little uniformity as to superelevation rates, just as there was little consistency in minimum radii for curves. Empirical controls were generally applied, with maximum cross slopes of about 10 percent.
A BPR study in 1920 focused attention on the necessity of a tire-pavement friction factor for superelevation design and also noted that a spiral curve or transition length was needed at each end of a curve section for the change from normal pavement crown to a superelevated section.[24]
One of several conclusions resulting from a long series of studies on tire-pavement skidding relationships conducted by the Iowa State University was that the maximum permissible speed as used in designing curves should not exceed that for which a useful side-friction coefficient of 0.30 was required to counteract centrifugal force. This conclusion was reached in 1934.[25]
In 1935, the BPR collected data from drivers across the country operating their own vehicles on curves of known radius and superelevation by asking them to report the speed at which they began to feel a side pitch outward. Analysis of these data resulted in a new design premise that safe operation on curves would be attained when the superelevation was sufficient to counteract centrifugal force for three-quarters of the expected speed, relying on side friction to supply the remaining horizontal resistance up to a maximum side friction factor of +0.16 at 60 m.p.h.[26] The speed concerned was advocated to be the “assumed design speed,” which was to be used as a basis for coordination of all alinement and geometric design values. The side friction factor to be used in calculating the minimum radius or the maximum rate superelevation did not enjoy the same degree of finality as the speed criteria, although the values used were apparently on the safe side. Research is continuing at the present time in an endeavor to discover pavement materials and construction methods to improve the skid resistant qualities of pavements.
In 1937 the BPR completed a highway curve design manual (later published as Transition Curves for Highways, 1940) embodying the above proposals. The manual presented data for 10 m.p.h. increments in design speed for all curve design features—curve radii, superelevation, curve widening and transition (spiral) curves. These concepts and design details were greatly needed and soon gained wide use, thus stabilizing, to a large extent, curve design practices throughout the country and nullifying for the time being any necessity for the AASHO Committee to concentrate its efforts on this subject.
Gradients
Common logic has always dictated that, from the traveler’s point of view, the most desirable route between two points is the one that is straightest and has the least rise and fall. In the days of wagon roadbuilding, circuity and indirection of alinement often had to be substituted for directness in order to obtain a suitable profile. The wagon roads were later converted to motor highways, frequently with little change in alinement and grade despite the fact that automobiles and trucks could negotiate grades steeper than the 4 to 6 percent commonly used for horsedrawn vehicles. This was done in the interest of economy, although the roadbuilders would have preferred a better alinement.
As the highway network expanded and more roads were built on new locations, particularly during the late 1920’s and the 1930’s, advantages were taken of the better gradeability of motor vehicles, and grades as steep as 9 percent were used sometimes to provide a straight alinement. Design with long tangents became commonplace and road distances were shortened by hundreds of miles in the aggregate. A BPR summary of practice in 1929 stated:
On main-line highways it is customary to adopt a maximum grade of 5 percent in gently rolling country and 7 percent in rough country, but it is no longer considered good practice to resort to sharp curvature in order to avoid grades steeper than 7 percent. If local conditions permit either a 7 percent grade with a sharp curve or a short 9 percent grade with a wider curve, the latter design is thought to be the better practice because it is safer for modern motor traffic.[27]
In the rolling terrain commonly encountered in the midwest and far west where roads were developed on section line locations, this type of design resulted in many hundreds of miles of “roller coaster” highway profiles. Design of profiles with frequent grades over 5 percent tended to minimize earthwork quantities, with only shallow cuts and fills. As traffic volumes, speeds, and truck loadings increased, the deficiencies of short sight distances and high-downgrade speeds proved that this type of profile was somewhat hazardous.
The alternative was a profile design of a railroad grade type, that is, long, easy grades with long flat vertical curves in conjunction with long horizontal tangents connected by gentle curves. Prior to 1930, any extensive mileage of such highway construction would have been out of the question because of the large earthwork quantities and attendant high costs.
The rapid mechanization of earthmoving equipment that began in the early 1930’s revolutionized construction methods and made feasible the construction of highways of a type that had heretofore existed only in the fanciful minds of design engineers. Tractor-drawn self-loading scrapers with capacities of 12 cubic yards came upon the scene for the first time. Pneumatic tires had been improved to such an extent that they could be used on heavy trucks, thus affording sufficient flotation to operate on newly placed embankments and made long hauls a routine operation. The roadbed of the then-modern highway, even though it was only two lanes in width, afforded sufficient room for turning and maneuvering. When compared with unit construction costs for the earlier years, earthwork became the best bargain in the entire roadbuilding operation.
Connecticut’s route 11 preserves the natural rock formations and vegetation by varying the width of the median.
The trends toward steeper gradients was arrested, if not reversed, by these developments. However, it was not until 1950 or later that a consensus was arrived at and anything approaching standardization of maximum grades was developed by AASHO.
Design Effects on the Environment
Highway construction inevitably left scars upon the landscape. These were not particularly objectionable until the era of heavy cuts and fills and relatively wide roadbeds that accompanied the design concepts of the 1930’s. Most State highway departments had programs for roadside development prior to that time, some as early as 1912. These programs consisted largely of planting trees and shrubs. These were reasonably successful prior to 1930 but were not adequate for the true automobile road. As one writer described the situation in 1936:
The quite common belief that the adoption and execution of a mere beautification program for our highways satisfies even the most obvious requirements and potentialities of roadside development is both incorrect and disheartening. We must sadly admit that much of our present so-called roadside improvement is little more than a landscape hair-cut or perhaps a horticultural manicure.
What is now most needed is to set up a better type of organization with engineering and landscape departments properly coordinated and working in harmony.[28]
This advice was heeded, at least in part, but the demands for more miles of paved highways and for widening and straightening existing ones continued to take the lion’s share of the highway dollar. Much credit is clue the National Park Service for their insistence upon the incorporation of esthetic quality in the design of national parkways. An outstanding example of the good results that could be produced by the joint efforts of landscape architects and highway design engineers may be found in the Mount Vernon Memorial Highway which was designed and constructed by the Bureau of Public Roads for the Park Service during the period 1929–1932.
Credit is also due to the membership of two committees within the highway fraternity—the Committee on Roadside Development of the American Association of State Highway Officials and a similarly named group in the Highway Research Board. Initially in 1932, these two groups were established as a joint committee but were separated in 1939 under their parent organizations. The term “complete highway” was coined by these committees to describe succinctly the importance of blending into a highway the fundamental elements of design, construction, and maintenance. The complete highway had to incorporate utility, safety, beauty, and economy to satisfy this very sound concept. Highway beautification was also given a boost by the National Recovery Act of 1933, which included among its objectives the landscaping, with Public Works funds, of a moderate mileage of main roadsides. The rules and regulations governing the use of these funds required that at least one-half of 1 percent of each State’s apportionment should be devoted to this type of improvement. Thus, a total of approximately $2 million was set aside for pioneering work which had for its ideal the conversion of unsightly roadsides into attractive areas bordering roadways made safe for those traveling upon them.
The Federal-Aid Highway Act of 1940 was a further landmark in legislation in support of better looking and safer roadsides. Section 11 of this act authorized the use of Federal funds for “. . . such roadside and landscape development, including such sanitary and other facilities as may be deemed reasonably necessary to provide for the suitable accommodation of the public . . . and . . . likewise . . . the purchase of . . . adjacent strips of land of limited width and primary importance for the preservation of the natural beauty through which highways are constructed . . .” The Highway Beautification Act of 1965 further liberalized the use of Federal funds for roadside improvements, such as control of outdoor advertising and the control of roadside junkyards.
Practical Applications
Returning for a moment to the Mount Vernon Memorial Highway, one reason for its attractive appearance was its gentle sinuous alinement; there were no long tangents and no abrupt, short radius curves. Yet, the highway formed a reasonably direct route between termini. It was the antithesis of the type of alinement that was evolving for conventional two-lane highways which, by their very nature, necessitated that vehicles encroach upon the opposing lane of traffic in order to overtake and pass slower vehicles. Straight alinement with good sight distances was thus a requisite of a good, safe two-lane road.
By way of contrast, the Mount Vernon highway was a four-lane highway on which faster cars could overtake and pass slower ones without having to encroach on the lane for oncoming traffic. Thus, passing sight distance was not important. The curves on the Mount Vernon highway were of longer radius than those generally used on two-lane roads, and there was no necessity for the motorist to vary his speed in traversing the entire length of the route. One curve has been said to be over 2 miles long. All curves were provided with spiral transitions.
This departure from conventional curve design was achieved through the use of a flexible spline of the type used in ship design, and this was one of the early applications of the method. A prerequisite of this method of location and design is a topographic map of fairly large scale and of sufficient width to include all alternative locations for the selected routes. As has been mentioned, aerial photographs were very helpful in this regard, and this project was one of the first to use this technique. The methods for determining elevations from the two-dimensional photographs were crude and lacked precision, and there were other problems to be overcome, but the benefits of seeing the vegetation, the drainage courses, the configurations of the ground, and the land uses as they appeared in nature were tremendous.
Rapid strides were to be made in photographic techniques and in the development of equipment for interpreting and photogrammetrically plotting topography by use of contours on topographic maps entirely adequate for precise highway location and design. Improvements were to continue. Looking ahead, the Federal-Aid Highway Act of 1956 authorized “the use of photogrammetric methods in mapping, and the utilization of commercial enterprise for such services.” By 1960, the accuracy and practicality of aerial survey methods were so well recognized that contractors were willing to accept payment for earthwork computed on the basis of cross sections measured photogrammetrically using aerial photographs taken before and after the construction work was performed.
An arterial highway of the 1940’s with uncontrolled access. An early form of channelisation helped traffic movements, but the danger from cars backing out onto the highway still remained.
At the same time that new techniques were being discovered or improved, new concepts and principles were being recognized. One of these, soon to be exploited, was the principle of control of access. It would be difficult to say when the first public highway was planned with the express intent of excluding abutting property owners from access to the road for the purpose of protecting and preserving the operational character of the highway. One of the earliest examples of a controlled access highway is the Bronx River Parkway in New York. It was designed about 1914 and completed approximately 10 years later. It has been said that this project was conceived as an attempt to protect the historical old Bronx River, but as studies developed, it was found that protection was feasible only if the land on both sides were purchased in fee. Once the land was purchased, it appeared desirable to use it for park purposes and then for a parkway to relieve the congestion on the heavily traveled north-south streets in the area. Regardless of whether this controlled access parkway came into being by accident or whether by intent, it stood for half a century as a lasting proof of the value of controlled access. Whereas other roads of much higher standard built decades later have since become obsolete because of roadside interference, the original Bronx River Parkway retained all of its beauty and utility until the pressures of continued traffic growth necessitated reconstruction during the 1960’s. Much of this beauty was retained in the reconstruction process.
The example set by the Bronx River Parkway and other early controlled access highways went largely unnoticed for many years, mainly because one of the primary functions of highways at that time was to serve the abutting property, not to isolate the road user.
During the period 1930–1940, highway designers became aware of the adverse effect that unregulated access could have upon highway traffic service. Strip commercial development in the vicinity of towns and cities was particularly troublesome, Vehicles turning into and out of roadside businesses created congestion and caused accidents. A few far-sighted individuals recognized the principle of control of access that had been pretty much dormant throughout the 1920’s. Interest was not limited to the United States. In 1933, the first spade of earth was turned on a new system of highways for the German Reich. These autobahnen, with full control of access and designed for speeds up to 120 miles per hour, were destined to exert considerable influence on American design criteria.
The roadbuilders in the United States demonstrated, however, that they were not dependent on the German engineers for leadership in designing and constructing freeways. Between 1934 and 1940 the Merritt Parkway was constructed, extending the parkway and controlled access features of the Westchester County, New York, parkway system across Connecticut. During its first full year of operation, it handled an average of 20,000 cars per day.
The Merritt Parkway near Darien, Conn.
The need for a traffic artery capable of handling similar traffic volumes between Los Angeles and Pasadena led to the start of the six-lane Arroyo Seca Freeway in 1938. It had two 35-foot roadways with a 6-foot curbed median.
The Pennsylvania Turnpike, completed in 1940 from Irwin to Carlisle as a toll facility, was the first rural freeway of notable length in this country. It was a four-lane divided highway, except for several tunnels through mountains where the width was narrowed to two lanes. Noteworthy features in the design standards were gentle alinement and relatively flat gradients suitable for high-speed travel despite the ragged terrain traversed by portions of the route. Also, consistent with anticipated high-speed travel was a design requirement for speed change lanes 1,200 feet long at points of access.
These and the several other freeways in use by 1940 met with immediate acceptance by the motoring public. The freeway era was born.
The Era of the Freeway: 1940–1976
Design Standards
Concentrated thought and effort toward a nationwide system of controlled access highways, both urban and rural, began to gather momentum about 1940. However, it would be totally erroneous to leave the impression that no further advances were made in the design of conventional highways after that date. It is generally true that the advances were in the nature of refinements of the basic concepts and principles enunciated in AASHO’s seven published design policies. These refinements generally took the form of more generous dimensions in such elements as 
AASHO prepared concise, abbreviated design standards for the several classes of highways, starting with those for primary highways in 1941. Design and construction standards for secondary and feeder roads and for the Interstate System were published in 1945. These have since been expanded and upgraded, as necessary, for the three general categories of roads. The classification terminology has been changed somewhat with the passage of time.[N 1]
These standards have since been approved by the Federal Highway Administrator for application on Federal-aid highways and are the specific controls for the design of such highways. Many States have also adopted them in their exact form for application on highways that are off the Federal-aid systems. Other States have promulgated standards of their own for application on their various classes of State highways. Where found to be in reasonable conformity with AASHO standards, these have been approved by the Federal Highway Administrator for use on the Federal-aid systems.
Getting AASHO standards approved has not always been accomplished without dissension and debate. When a pavement width of 24 feet was proposed for heavily traveled two-lane roads in 1941, for example, some officials, accustomed to building lanes only 9 or 10 feet wide and having been stung by the bad accident experience on three-lane roads, were fearful that drivers would mistake the 24-foot roadway for a three-lane road. Other States were deeply concerned by the added costs for higher standards.
- ↑ Current versions prepared by the Committee on Planning and Design Policies are Geometric Design Standards for the National System of Interstate and Defense Highways—1967; Geometric Design, Standards for Highways Other Than Freeways—1969; and Geometric Design Guide for Local Roads and Streets—1970.
Rural Design Policies
Highway design policies are general procedures and controls which are less specific than design standards, often with a range of acceptable values, and which are officially adopted or accepted for application in the design of highways. In 1950, the seven separately published design policies were reprinted and bound as a single volume under the title, Policies on Geometric Highway Design. Parenthetically, the final one of the policies, published in 1944, dealt with interchanges and grade separations and should properly be identified with freeway development to a greater extent than with conventional highways. The policies were updated and republished in 1954 as A Policy on Geometric Design of Rural Highways, the “Blue Book.” This publication broadened considerably the earlier work, largely as a result of research efforts in the field of traffic operation. The Highway Capacity Manual, published by the Highway Research Board in 1950, supplied material on the relation between highway capacity and roadway characteristics. This information was incorporated in the 1954 rural design policy and has been invaluable in aiding the designer to better fit the highway to traffic requirements.
The results of studies of truck speeds on grades as related to motive power and load carried also became available after 1940 and were used in selecting values for control gradients for the design of the several classes of highways in different types of terrain. One solution to the problem created by slow moving trucks on grades was the provision of an added lane in the uphill direction for use by trucks. These have become known as “climbing lanes.” Criteria for climbing lanes were included in the 1954 policy.
The 1954 design policy also enlarged upon the design speed concept and included tables showing the relation between design speed, degree of curvature, rate of superelevation, and needed length of spiral transition. The subjects of freeway design and interchange geometries were discussed to the extent that the state-of-the-art permitted.
In 1965 the rural design policy was again brought up to date and republished under the same title.
A Need to Expand the Highway Network
As efforts to improve and expand the highway network continued, it became increasingly apparent to highway administrators and lawmakers alike that portions of the predominantly two-lane rural system of highways were becoming severely strained by the ever increasing traffic burden. Urban arterials were becoming choked by intensified commercial development coupled with rapid traffic growth. Inadequacies in the form of low travel speeds, low capacities, and high accident rates were clearly evident to those at even the highest level of government. At the same time, there was an awareness that transportation within cities was a national matter rather than a local problem. As a means of defining the scope of the problem and of developing remedial measures, President Roosevelt, on April 14, 1941, appointed the National Interregional Highway Committee to “. . . investigate the need for a limited system of national highways to improve the facilities now available for interregional transportation, and to advise . . . as to the desirable character of such improvement. . . .”[29]
The product of the Committee’s efforts was the celebrated report Interregional Highways, submitted to the President on January 5, 1944. The ultimate accomplishment was the incorporation in the Federal-Aid Highway Act of 1944 of a provision for designating a national system of highways and a further provision for the expenditure of Federal-aid highway funds in urban areas.
Highlights of some of the recommendations of the report as to locating and designing the system were:
- The system would be both urban and rural in extent.
- Roadways and structures would be designed to serve vehicles of the types and numbers to be expected 20 years from the date of construction.
- Intersections with crossroads and railroads would be separated in grade.
Traffic jam at intersection of U.S. routes 3 and 20 in Boston, Mass., in the early 1910’s.
Rural bridges became dangerous bottlenecks when they were narrower than the approach highway, often making two-way traffic hazardous or impossible.
- Rural sections would be designed for safe travel at a speed of 75 m.p.h. in flat topography; urban sections for 50 m.p.h.
- Traffic lanes would be 12 feet wide.
- Shoulders would be 10 feet wide except in mountainous topography.
- Embankments 10 feet or less in height would have side slopes no steeper than 1 foot vertically to 4 feet horizontally.
- The roadway width on bridges would be at least 6 feet greater than the width of the pavement of the approach roadway; on short bridges the roadway width would be as great as the width of approach roadway, including shoulders.
There were many other details for the design and construction of the system, including such items as signs and markings, lighting and landscaping. A discussion of the principles of landscape design occupied three and one-half pages of the report. It was a complete text within itself.
The design criteria contained in the report were recommendations only. The 1944 legislation authorizing the designation of the National System of Interstate Highways made no reference to the standards to be used in the design and construction of that System. This was to come later.
Enabling legislation for financing and constructing the System was enacted in 1956. This legislation contained the unique provision that the standards for the System would be those approved by the Secretary of Commerce “. . . in cooperation with the State highway departments.” Thus, the continuance of the voluntary cooperative efforts of the States and the Bureau of Public Roads in formulating standards for the other highway systems for the past 20 years was firmly assured and made applicable to the Interstate System.
AASHO responded to the requirement of the law by adopting standards for the System within 3 weeks after passage of the Act. Many of the recommendations of the 1944 Interregional Highways found expression in these standards, which were approved by the Secretary of Commerce in accordance with the law.
Urban Design Policies
The Committee on Planning and Design Policies was well aware, as it always had been, that bare-bone standards are not enough to assure the design of a safe, utilitarian highway that is esthetically pleasing and economical to construct and maintain. They recognized a need for a more casual and philosophical discussion of the principles of freeway location and design. The Committee was also confronted with the challenge of developing guidelines for the utilization of Federal-aid funds in the construction or improvement of city streets and highways.
The Committee chose to combine these objectives and to develop a policy on urban arterial highways, which would, of course, include freeways as well as conventional arterial surface streets. This effort culminated in the publication, in 1957, of A Policy on Arterial Highways in Urban Areas, the “Red Book.” When work began on this policy, there were few freeways in existence, and experience in their design and operation was limited. Nevertheless, the vision and foresight of the Committee members was sufficient to result in a thorough and comprehensive text on this subject, as well as on the more conventional types of streets and urban highways. The discussion of interchange types and configurations was particularly exhaustive.
Experience in the application of the 1957 urban design policy was generally favorable, but after a period of years, operational deficiencies began to develop in many urban freeways built in accordance with the 1957 doctrine. In large measure, these deficiencies were attributable to traffic loads far in excess of those that were anticipated at the design stages. Nevertheless, many of the operational problems would have been alleviated had some of the dimensional values and configurations been more generous. Accordingly, the policy was updated and republished in 1973 as A Policy on Design of Arterial Highways and Urban Streets. In the process of revision, new sections were added on urban transportation planning and on arterial route location.
Design standards and guidelines have undergone only minor modification and upgrading since adoption of the first editions of the two geometric design policies, rural in 1954 and urban in 1957. Such changes as have been made are attributable to two principal factors: (1) Travel speeds have continued to climb, necessitating adjustments for all design features, and (2) vehicular silhouettes have been lowered, requiring natter highway profiles to provide the necessary sight distances for avoidance of accidents.
Clearances to roadside obstacles and the moderately flat side slopes that were entirely adequate for the travel speeds prevalent before the mid-1950’s were found deficient when measured against the speeds and other operational practices of the seventies, as is apparent from an examination of speed trends on main rural highways. Consequently, more liberal dimensions have been incorporated in the standards and above minimum design is the rule rather than the exception.
It may be truthfully said that throughout the history of highway design devekmment, the highway user, in the collective sense, has dictated the character of the highway by his manner of operation on it and by the extent of his willingness to pay, through road user imposts, for roads that would sustain that type of operation. It remained for the design engineer, working in concert with the research engineer, the landscape architect, and the economist, to determine the type of operation demanded by the vehicle operator (now and in the future), the design characteristics of a highway system that would safely support that type of operation without being unnecessarily extravagant, and the probable revenues that would be available. It would not be a gross exaggeration to say that every highway project has represented a compromise between the ideal in design characteristics on the one hand and economic reality on the other. This is likely to remain the case in the future. Design For Safety
While not always indicated separately, safety in highway vehicle operations has been an important objective of highway design since the beginning of the highway development programs. The identification by the 1920’s of the need for clear stopping distances, for widths to pass without major pullover, and for usable curve radii led to early efforts to determine and incorporate such appropriate features in the next highway improvements. With reasonable rapidity, the design engineers accepted the identified highway safety needs, developed newer design controls reflecting them and put them to use. In retrospect over the last 50 years, it should be said that designers never were able to catch up with nor were the construction programs able to provide the highways needed for the rapidly changing vehicles, volumes, speeds, travel habits and driver attitudes. The history of highway design control development shows a progressive series of adjustments that reflect the concerns for highway safety evolving from the latest experiences and accident data. The same process continues today.
There is space here only to indicate a few examples of design controls and practices that were pointed toward better safety. In 1938–1944 initial design policies of AASHO had many such items. The 1941 primary standards preamble urged that in using the design values, “unquestioned adequacy rather than strict economy should be the criterion.”[30] The overall concept of design for expected volumes, for the character of traffic and for an assumed design speed was a major effort for highway safety. The enumerated values and controls for lane width, shoulders, use of a dividing median, pavement crown, curve radius and superelevation, minimum sight distance, safety passing sections, guardrails where roadway slopes were steep, appropriate intersection details and layouts, and for grade separations with interchange ramps all embody features patterned for safe vehicle operations. The 1954 rural design policy book refined these and provided additional details; advocacy of controlled access design was a major addition. The 1965 revision expanded treatment of these details, primarily in the realm of the higher volume and higher speed operations which were then being experienced. Both of these editions keyed into the separately developing standards for the several types of traffic control devices which have a high safety orientation. Both the 1957 and the 1973 urban highway policy books reflected design features needed for the higher volumes and greater land space restrictions in urban areas. They particularly stressed the design details of grade separations, interchange patterns, frontage roads, and practical geometries relative to expressways and freeways.
The Snow Road Bridge over I-496 near Lansing, Mich., is an example of modem safety design. Slanted back slopes and the absence of side piers on either end make safe vehicle recovery possible.
The 1956 Interstate standards made a further breakthrough in stating positive requirements for design with full access control throughout that System and for use of higher realm geometric design values as needed for operations at speeds of 60 m.p.h. and above.
During 1959 and again in 1966, a special AASHO Safety Committee conducted a nationwide survey of highways which resulted in published reports enumerating design details to provide greater safety. The second of these, Highway Design and Operational Practices Related to Highway Safety, 1967, soon known as the “Yellow Book,” included recommendations on the current highway design and practices to attain a higher degree of safety in both State and local agency actions. Responding, Federal policy directives called for utilization of the report recommendations on all project plans designed for a speed of 50 m.p.h. or more and a corrective program to apply the findings on existing highways. The report was updated and reissued in 1974 with the same title.
One of the newly advocated features from these studies was the “clear roadside” concept, which by 1966 had been started in a few States. During the early 1960’s, it became evident through accident analyses that about one-third of all accidents were single vehicle, run-off-the-pavement types, a high proportion of which proved to be of high severity on collisions with an object on the roadside, such as a rock, large tree, or steep cut slope. Some of these objects were highway elements such as culvert headwalls, rigid sign posts, lighting standards, bridge piers, etc. Better design and correction programs for safety were needed to provide traversible roadsides of a width well beyond the shoulder that was free of all formidable objects and reasonably flat and rounded so that off-roadway drivers would have a chance to recover control of their vehicles. Actions taken included lengthening culverts and overcrossings, moving sign supports or using the breakaway type, eliminating protruding drainage inlets, flattening and rounding roadway slopes, and providing tested-types of crash cushions at bridge abutments and between diverging highways and guardrails in front of essential roadside objects and as central barriers in narrow medians. Main highways of new design now are being developed in this manner and corrections made to the extent practical on existing highways. The cost effectiveness of some of these features remains moot on low and intermediate volume highways and further studies are continuing for their more widespread application.
Traffic Control Devices
Traffic control devices are the several elements provided on highways to advise, guide, warn, regulate or otherwise inform the vehicle drivers. The basic types of devices are signs, pavement markings, roadside delineators and traffic signals. The 1971 Manual on Uniform Traffic Control Devices was approved by the Federal Highway Administrator as the national standard for all highways open to public travel. This Manual presents essential standards for design, location, installation and operation of all forms of traffic control devices and is the culmination of over 50 years of progressive development of the devices and national standards for their detailed and uniform use.
The history of these developments can be briefly reviewed in two stages. First are the series of individual and scattered instances to conceive and use some form of device. Second are the organized efforts to develop and attain acceptance of standards for national uniformity.
Early Traffic Controls
In 1745 stone markers were placed between Trenton and Perth Amboy on the principal highway between New York and Philadelphia. They were installed at 2-mile intervals and at intersections with other public roads. A public subscription was made to pay for them. In 1763 the Boston Post Road similarly was marked with mileposts.
The Pennsylvania State authorizing act in 1792 for the Philadelphia to Lancaster turnpike included requirements for mileposts and directional signs.
By 1902 some cautious automobile touring had begun, and the adventurers were losing their way. Often there were no signs at all, or where there had been signs, many had toppled over and been broken or faded beyond readability. As early as 1905, extensive signpost work was performed by the Buffalo Automobile Club in New York State, In the next few years, auto clubs across the country undertook the task of a basic directional signing on the principal highways within their areas. Despite their local efforts, there were no national or long route installations.
In 1913 the Lincoln Memorial Highway Association was organized and funds collected toward promotion and construction of a central east–west highway across the country. While the construction phase largely became promotional assistance to the State highway agencies, the Association shortly put into effect an entire route marking along existing roads. Painted red, white and blue band markings and symbols were placed on utility poles, clearly designating the chosen route. This example generated widespread activity by many motorist and local clubs to similarly, but distinctively, mark a selected and named route and foster its improvement. By the 1920’s, there were some 250 name routes, each with characteristic color bands marked on roadside poles. In some cases, several such routes overlapped, resulting in totem poles of multicolors. These color bands and symbols were gradually replaced by route numbers as the States installed signs designating them.
The Automobile Club of Maryland posting directional and mileage signs.
The widespread use of stone mileposts did not begin until the early 1920’s when mileposts appeared on the roads of a few States in the form of concrete marker posts. Gradually the mileposts began to be replaced by signs indicating mileages to places ahead to aid travelers. The rapid expansion and drastic changes in our Nation’s highway system beginning about 1910 were reflected in significant modifications in highway markings. The realinement and abandonment of roads, together with construction of new highways, made many of the old mileage signs virtually useless, and they were gradually replaced by signs displaying point-to-point distances and route numbers based upon uniform statewide highway numbering systems. In addition, travelers were greatly aided by the widespread production and distribution of tourist maps that made use of readily identifiable landmarks, as well as route markers and signs. This increased availability of other devices for the guidance of travelers resulted in a marked decline in the use of mileposts, except in a few States and on turnpikes.
An early railroad crossing warning.
Immediately after World War I, as the States engaged in a greatly speeded-up road development program, it became apparent that a simplified and adequate marking system was a necessity. In 1918, Wisconsin decided that proper names could not be applied to an extensive network of highways and so developed the route marker and the numbering method of designating its highways and for directing travel over them. The shape selected for its route marker signs was a triangle with the apex down, using black and white colors. The triangle showed “State trunk highway,” the route number and the State name. Also Wisconsin placed directional and distance signs for the trunk system and for lesser roads.[31]
In 1921 Minnesota established its trunk highway system and promptly installed full-scale marking and signing. Minnesota adopted a star shaped design, with lemon yellow and black colors for all official route signs on the system and suggested white and black for signs placed by other jurisdictions.[32]
In 1911 the road commissioner in Wayne County, Michigan, ordered that a white line be painted down the center of every bridge and curve under his authority. Later, he carried the idea to its logical conclusion and painted the centerline along all the highways. The obvious benefits of the centerline strip were eventually realized, and the striping spread to all highway agencies.[33]
In 1915, a Detroit police official designed the first stop sign. When the first installation proved effective, the city promptly allocated funds to make six major streets through thoroughfares, by placing stop signs on intersecting streets.[34]
Traffic signals were developing in this same decade, first with the hand operated semaphores, then a motorized version that was patented in 1910. The first use of electric traffic signals was during the period 1912 to 1914 with Cleveland, Salt Lake City, and St. Paul all claiming their use to be the first.[35] In 1920, the first three-colored traffic signal light to control street and highway traffic was installed in Detroit.[36] The first four-way three-color signal was installed in Detroit in 1920.[37]
The first traffic control tower, located within an intersection, was set up in 1917, at a main Detroit crossroad.[38] Similar towers also were used about the same time in New York City. 
These route and mileage markers aided travelers passing through Jackson, Mich.
Uniformity Through Standards
Late in 1922, three State highway department officials from Minnesota, Wisconsin, and Indiana joined in a trip through several States to try to work out some basis for uniformity in the signing and marking of their highways. The trio’s findings were reported at the 1923 meeting of the Mississippi Valley Association of State Highway Departments. That body agreed on them as a uniform signing and marking plan for the member States and passed its recommendations on to AASHO. Two years later the system of signs and markers became the basis for the first national standards.[39]
The Mississippi Valley Association established distinctive shapes for the several classes of signs, namely a circular railroad crossing sign, an octagonal stop sign, a diamond-shaped slow sign, and a distinct route marker to be individually designed by each State. All of these signs were to have a white background with black lettering and border. With the exception of the route markers and the rectangular information sign, all were to be two feet square or 2 feet across. Within a year Wisconsin, Minnesota, Indiana, and Michigan were installing signs on their State systems largely in harmony with the Association plan, and other States in the Association were making plans to do so promptly. A Minnesota Highway Department Manual of Markers and Signs was completed in April 1923.
The report of the National Conference on Street and Highway Safety in 1924 by its Committee on Construction and Engineering stated that:
Proper signs and signals are essentia] to the safe movement of traffic on any street or highway. . . . Signs should be uniform for a given purpose throughout the United States. . . . It can be assumed that the Federal-aid signs . . . will lie uniform in every state and will point the way to state and county highway authorities to follow the same standards. . . . All signs should be simple, with the least amount of wording necessary to make them readily understood, depending mainly on distinctive shapes, symbols and colors.[40]
The Conference report recommended that color indications, which would not be used for any other purpose, should be red for “stop”; green for “proceed”; yellow for “caution” at curves; some special cautionary color indications should be used at crossroads; white letters or symbols should be used on the red or green background, and black on the yellow. Distance and direction signs should be black and white.
Other recommendations were :
Railroad crossings remaining at grade should be safeguarded in every reasonable way. Standard warning signs and pavement markings should be used to mark the approaches to all public railroad crossings. ***** Rural highways should be marked with a white center line on curves, at and near hill crests, at irregular intersections, and at any other points where safety requires that motorists keep strictly to the right. No parking even off the traveled roadway should be permitted opposite these white lines. White center lines should not be used on straight level sections of highway or street except at highway, street or railroad crossings. Black center lines on straight sections of highways are desirable.
Pedestrian lanes should be marked on the pavement at busy intersections.
Objects near the roadway, such as curbs, poles, fences and rock surfaces, should be painted white. Obstructions, such as columns and curbs, at the centers of underpass, should be striped diagonally black and white.[41]
By 1922, fourteen towers of this type had been installed at main intersections in Detroit, Mich.
STANDARD DANGER SIGNS—1919
At the annual meeting of AASHO in 1924, its Subcommittee on Traffic Control and Safety presented recommendations for standard signs and markings based largely on the work of the Mississippi Valley Association, but incorporating at least one new feature, a color code to distinguish the several types of signs. Acting on an AASHO resolution, the Secretary of Agriculture appointed a Joint Board on Interstate Highways in March 1925 with 21 members from State highway departments and 3 from the Bureau of Public Roads. In October, the Board made its report covering the proposed interstate highway network, the route numbering system, and a comprehensive set of sign designs. The system devised for numbering the interstate network of highways (now called the U.S. Numbered Highways) used even numbers for east and west roads and odd numbers for north and south roads. Long distance routes which might be connected entirely, or nearly so, across the country were given multiple numbers of 10. The principal north and south routes were given numbers such as 1, 5, 11, 15. Other numbered routes could be used for shorter lines between the main designated routes.
The Joint Board sign standards followed closely the 1924 Conference recommendations with the addition of the now familiar “U.S. shield” marker for the new network. A yellow background was adopted for all caution and danger message signs, including the stop sign. There were no recommendations on luminous or reflectorized signs.
The Joint Board report was submitted to the Secretary of Agriculture and approved by him November 8, 1925. It was accepted at the annual AASHO meeting a few days later and subsequently adopted by letter ballot of the State highway departments.
During the following year, AASHO developed detailed sign standards and published the first edition (1927) of the Manual and Specifications for the Manufacture, Display, and Erection of U.S. Standard Road Markers and Signs. This Manual set forth the design and use of each type of sign and illustrated most of the approved signs. It listed a series of working drawings of standard signs and alphabets that had been prepared for distribution by the Bureau of Public Roads. Also, it contained detailed specifications for materials and manufacturing of various types of wood and metal signs. This 1927 Manual was the first national rural manual on traffic control signs and markings. A 1929 second edition authorized the use of a luminous element mounted below a standard sign on the same post or on a separate mounting in advance of the standard sign framed with a background of the same shape and color as the standard sign that it supplemented. A 1931 revised edition added a number of new signs, including a new design for junction markers.
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The standardized U.S. route shield.
Two years after the 1927 AASHO Manual, other officials gathered for urban sign standardization. The National Conference on Street and Highway Safety, recognizing the need for greater uniformity in street traffic signs, signals, and markings, accepted an offer of the American Engineering Council to make a national survey of existing conditions and to prepare a recommended practice. The resulting report accepted most of the AASHO Sign Manual standards, but with some exceptions and qualifications. The recommended “stop” sign was to have a yellow background, but the letters were to be red. Eighteen inches was to be the standard outside dimension instead of 24 inches because the generally slower speeds in cities did not require large signs for visibility and the smaller signs would occupy less space. Parking regulation signs and other signs applicable to city use were added. These parking signs were to be 12 by 18 inches with a white background. Red letters were to be used on “no parking” signs, and green letters where limited time parking was permitted. Pedestrian restrictions were to be shown in blue letters on a white background, and other restrictions in black on white.
The urban recommendations also included subjects not dealt with by AASHO, such as traffic signals, pavement markings and safety zones. The signal recommendations included terms, systems, control types, colors, specifications, beacons, and “wigwag” and flashing light train signals. The marking recommendations included pavement lines, railroads, identification, curve lines, words, pavement inserts, paint, buttons, and markings on vertical elements such as obstructions and railroad gates.
The Council’s report was submitted and approved by the Third National Conference on Street and Highway Safety in 1930. Then there were in existence two national manuals, one for rural use and one for municipal use, with some significant differences between them. It was apparent to all that a single manual covering all traffic control devices would be desirable. Soon a Joint Committee on Uniform Traffic Control Devices was established, represented by AASHO for the rural manual and the National Conference for the urban manual. At the first meeting of the Committee, it was agreed that certain details should be thoroughly investigated before completing the joint sign manual. A project to investigate visibility and legibility of several alternative color combinations by day and night, with and without reflector buttons, was arranged with the Bureau of Standards by the Bureau of Public Roads. The Bureau of Public Roads also made delay studies at traffic control signals. With these and other data, Committee compromise adjustments were worked out and a combined manual was developed. The new manual was approved by the Secretary of Agriculture as the standard code for application on Federal-aid highways; it was published in 1935 as the Manual on Uniform Traffic Control Devices for Streets and Highways (MUTCD).
The new Manual dealt comprehensively with the whole field of traffic control devices. Signs were divided into three classifications—regulatory, warning, and guide. The stop sign retained the yellow background and red letters were accepted as an alternative to the black letters. The railroad crossing sign was a crossbuck to closely resemble the standard railroad advance warning sign. The Manual included new self-evident symbols to indicate crossroads, Y and T intersections and side roads. The section on traffic signals supplied long needed guidance on the number, meaning, and arrangement of signal lenses. Green was specified for go, yellow for caution, and red for stop. The Manual provided that each signal should have three lenses since the function of the yellow light could not be attained with a signal having only two lenses.
In 1938 the Joint Committee reexamined the Manual and recommended numerous updating revisions, and a supplement was issued in February 1939.
As one of the aftermaths of the Pearl Harbor attack on December 7, 1941, problems of wartime traffic loomed, especially the altogether new possibility of having to move traffic under blackout conditions. The Joint Committee was revived, this time including the Institute of Traffic Engineers as a member. The “War Emergency Edition” of the Manual, published at the end of 1942, was a condensed version of the previous edition, modified only to deal with blackouts and make certain wartime concessions, such as permissive use of white pavement markings instead of yellow because of material shortages.
In 1944 the Joint Committee agreed that the Manual must be entirely rewritten to include the needs of changed conditions. In 1948, after a wide review, the final draft was approved by the Joint Committee’s three sponsoring organizations and by the American Standards Association. That year, also, the National Committee on Uniform Traffic Laws and Ordinances was made a member of the Joint Committee replacing the National Conference on Streets and Highway Safety.
Some of the more significant changes in the 1948 Manual were: a diamond-shaped sign was prescribed for all warning signs (except the circle for the railroad crossing); a new intersection route turn symbol sign was provided; a new “advisory speed” sign was included for use with any warning signs; and the growing need for larger signs was recognized and some of the minimum sizes were increased.
A major change and innovation was made in the next Joint Committee Manual revision of 1955. The stop sign was changed from black on yellow to white on red. The yield sign came into being in the now familiar equilateral triangle with one point downward and used black lettering on yellow as were other warning signs.
In 1955 the MUTCD did not include material for the signing and marking of freeways. The National Joint Committee on Uniform Traffic Control Devices began drafting an extensive addition to the existing manual in 1956 to cover expressway signs as called for by the 1956 Federal-Aid Highway Act. Also in 1956, the American Association of State Highway Officials set up a subcommittee of its traffic committee to develop an Interstate Manual that would “incorporate the best experience from all the higher type toll roads and freeways.” The resulting Manual was adopted by AASHO in 1958. In February 1958, the Manual for Signing and Pavement Marking of the National System of Interstate and Defense Highways was approved by the Bureau of Public Roads. A second edition of the Interstate Manual was published in 1961 and a slightly revised edition in 1962.
In the meantime, to provide broader representation, the Joint Committee, in 1960, had enlarged its membership to include the National Association of County Officials (now the National Association of Counties) and the American Municipal Association (now the National League of Cities). The Joint Committee published a new MUTCD in 1961, drawing heavily on the AASHO Interstate Manual for expressway and freeway signing. The new Manual had many refinements, changes in emphasis, and new applications of engineering and psychological research and experience in operations of the substantial mileage of high-speed limited access highways, particularly of toll roads. New features included the use of lowercase letters and green background on freeway directional signs, reflectorization or illumination for the background of overhead signs, and reflectorization of all pavement markings that have amplication at night. The 1961 MUTCD also included, for the first time, an extensive special treatment of traffic control devices for highway construction and maintenance operations and a group of special signs for emergency civil defense applications. Standards for traffic signals were modernized to keep up with technical advances in that field.
The 1966 Highway Safety Act provided for the first time that specific Federal safety funds be spent by local governments as a step toward reducing the number of accidents. The existing devices on all highways and streets were to be continually reviewed and upgraded. The Federal-aid requirement for uniform traffic control standards was extended to apply to all streets and highways. The Act provided funds for the improvements, which would ultimately place the Nation under one set of traffic control standards.
New signs, 1971. Many of the signs combine international symbols and word equivalents.
Using the Joint Committee as an advisory board, a new updated revision of the MUTCD was prepared by the FHWA staff. Issued as the 1971 Manual on Uniform Traffic Control Devices for Streets and Highways, it is now the national standard for all highways open to public travel.
The 1971 MUTCD incorporates significant changes in the wider use of symbols, which are international in character, on both regulatory and warning signs. A separate part covers traffic controls for school areas. A pennant shaped “no passing zone” sign and pentagon shaped school signs were added. For construction and maintenance work, the colors for the warning signs were changed to orange. In the pavement marking area, yellow was added as the color to delineate the separation of traffic flowing in opposite directions. Traffic signal updating included recommending a 12-inch signal face instead of 8-inch for arrows. It emphasizes that engineering study always is an important part of the application of detail standards.
Surfacing and Paving the Highways
Development of Pavement Design
The history of the development of highway pavements in the United States to the present stage is a series of incidents that are widespread both in time and place. Once roads advanced beyond the footpaths and horse trails, their improvements into paved highways were successive developmental efforts to match the available manpower, funds and natural resources against the changing needs of the types and extent of vehicular traffic for each area’s expanding commerce.
The first “macadam” surface in this country was constructed in 1823 between Hagerstown and Boonsboro, Maryland. Rocks were broken by hand so as not to exceed 6 ounces in weight or to fail to pass a 2-inch ring. The material was laid in three separate strata, the finished surface being 15 inches deep at the center and 12 inches at each edge. The surface was 20 feet wide.[42]
In the forested sections, plank roads were dominant for a period. The first plank road in the United States was opened to traffic in 1846 in Syracuse, New York. Advocates of plank roads made extravagant claims as to their superiority over macadam, and lumber being available, thousands of miles were built in many States during the following decade. In a few years, the public discerned that the life of any road is limited by the lasting qualities of the material of which it is built. It took about 10 years for the wooden planks to rot away, and the plank road era ended rather abruptly.[43]
Dust palliatives were applied on gravel and macadam road surfaces as early as 1898. Tar and asphalt were used as protective surface coatings and later as binders.
Bituminous pavements were constructed in Washington, D.C., and New York City as early as about 1870.[44] Bituminous macadam experiments began in Boston in 1906, and a 1-mile section was constructed there in 1907.[45]
Recovering from the ravages of the Civil War, the South needed a road surface that could be built and maintained at a small cost from local materials in general abundance. Sand-clay surfacing, because it was lower in cost, adequate for light traffic, less dusty and noisy, and more resilient than macadam, was the logical answer to the road problems of the South Atlantic and Gulf States in 1885. It was used widely in these areas.
The first brick pavement on a rural road was placed near Cleveland, Ohio, in 1893. The roadway was 32 feet wide, but the brick pavement, 8 feet wide, was placed near one side of the roadway, leaving the remaining width unsurfaced.[46]
Mention has been made of early macadam, brick, and bituminous surfaces and pavements on rural roads. By the 1890’s the heavy-load hauling demands in the large cities had led to construction of heavy street pavement sections of the types which had shown good stability. The horsedrawn drays hauling heavy loads on steel-rimmed wheels had pulverized all but the hardest pavement surfaces. Consequently, the main streets of the large cities were built very heavily and surfaced with granite blocks or hard paving bricks. Concrete bases were used on some. The minor business streets and residential streets were commonly of macadam or gravel. Asphalt paving, begun in the early 1870’s, was immediately popular because of its smoothness, silence, lack of dust, and ease of cleaning. By the 1890’s, many of the city streets were asphalt surfaced to gain these advantages.
Credit for first surfacing a rural public road with portland cement concrete is conceded to Wayne County, Michigan, where a 1-mile section was built in 1909. The pavement was laid in two courses, 18 feet wide and having a total depth of 6½ inches. The first course was made of 1–2½–5 mix of portland cement, sand and limestone 4 inches deep and the second course of a 1–2–3 mix of portland cement, sand and crushed cobblestone 2½ inches deep. It was laid in 25-foot sections.[47]
The heavy truck traffic during World War I inflicted widespread damage upon surfaces built during the preceding generations to carry horsedrawn vehicles. After the war, there was a public clamor for improved roads, at once and everywhere. Responsible engineers agreed that they lacked the essential information required for the design and construction of a nationwide system of paved highways as envisioned by the framers of the 1916 Federal Aid Road Act. This led to the formulation and carrying out of the continuing and extensive series of engineering research studies by the State highway departments, Bureau of Public Roads and related industry. The Highway Research Board, created in 1920, served to nationally correlate and disseminate the ensuing results of the studies, delving into the proper relationship between highway loads, road surfaces, and subgrades. The system of soil classification and analysis was developed, together with data on the many detailed characteristics of aggregates, materials and mixes under the varied field conditions and traffic loads. Traffic counting and weighing facilities were set up, and the procedures for predicting traffic volumes and loadings were developed and put into use in the late 1930’s.
As the numbers, sizes, weights and speeds of vehicles increased, highway officials struggled to provide pavements with adequate structural strength to meet the demands of traffic. Practices of highway design, construction and maintenance were progressively revised upward. Some of these revisions were the result of practical experience and some were based on results of engineering research. In the 1920’s, many highway engineers recognized that the supporting ability required in the pavement structure of roads was determined principally by the axle loads of the vehicles. There was also general recognition of the need for more factual information on all of the pavement and soil elements.
Design Road Tests
Beginning in 1920, several research projects using specially constructed test tracks produced significant advances in the science of pavement design and construction. In 1920, the Bureau of Public Roads expedited its field tests on the large circular track containing different pavement sections at Arlington, Virginia. Experimental roads were constructed in 1921 by the Columbia Steel Company cooperating with the California Highway Commission at Pittsburg east of San Francisco. The Bureau of Public Roads intensified its laboratory tests and initiated a countrywide field study of subgrade soils. Universities became beehives of research activity. One major effort to obtain such information was the Bates Experimental Road, a test conducted by the Illinois Division of Highways in 1922 and 1923 near Springfield, Illinois, on a 2½-mile roadway divided into 63 test sections of varied materials and design.[48] The test vehicles were trucks with solid rubber tires on which wheel loads were increased from 2,500 to 13,000 pounds as the testing progressed.
Within a few years, there were four principal types of findings: (1) Subgrade soil tests were developed which, together with traffic studies and other information, became valuable aids in pavement design. (2) The destructive impact of the solid rubber tire was isolated and overcome by the introduction of the softer pneumatic tire. (3) Agreement was reached on the 9,000-pound wheel load as a logical basis upon which to plan a long-term paving program. (4) The thickened edge design of rigid pavements was adopted. These findings soon brought about the adoption of laws regulating vehicle weights. 
The Baltimore-Washington Boulevard, was greatly damaged by World War I truck traffic
By 1930, the same stretch of road had been resurfaced with bituminous concrete and widened by the construction of two 10-foot concrete shoulders.
The Baltimore-Washington Blvd. (U.S. route 1) in 1975.
Aware of the need for uniformity of State motor vehicle weight regulations, the Governors Conference, in 1949, requested a study of the matter. This study resulted in a rigid pavement test road being established in Maryland, with participation by 11 State highway departments, the District of Columbia, the Bureau of Public Roads, auto manufacturers and the petroleum industry. The purpose of the testing program was to determine the relative effects of various axle loads and configurations on distress of rigid pavement. The findings supplied extensive data on the factors to be used in the design of rigid pavements.[49] Especially significant were those factors regarding support material characteristics.
The Western Association of State Highway Officials constructed a test road in Idaho in 1951 to aid in establishing load limits and to develop rational design methods for flexible pavements. A number of specially designed and constructed bituminous pavements were carefully observed under the repeated application of a number of selected heavy axle loads. The findings provided significant information on the materials and soil parameters for designing flexible pavements, especially in the western States where soil conditions are similar. Also the surfaced shoulder was found to contribute to the pavement structural stability.[50]
In 1955, the American Association of State Highway Officials undertook the AASHO Road Test at a selected site near Ottawa, Illinois, with the Highway Research Board accepting the responsibility of administering the project. Various heavy truck loads were operated on specially constructed pavement sections of both rigid and flexible types until they reached a failure stage. A vast amount of data was collected and analyzed, providing engineering facts for highway design and construction nationwide. In addition, the test findings were aimed at determining maximum desirable weights of vehicles to be operated on Federal-aid highways, including the Interstate System, and determining an equitable distribution of the tax burden among various classes of persons using Federal-aid highways.
The findings of the AASHO Road Test were summarized and prepared in the form of design equations and graphs and made available to the States in 1962, but it was not until 1973 that the data was updated and published by AASHO as Interim Guide for the Design of Rigid Pavement Structures. The equations and graphs incorporated the research data for design factors such as traffic, soil support and material strengths. The guide today represents the major current data available for broad application in designing pavements and is used by the Federal Highway Administration to measure the adequacy of the States’ proposed pavement designs for use on Federal-aid highways.
Present day pavements consist of layers of bituminous materials or portland cement concrete, plain or reinforced, with thicknesses ranging up to 10 inches; in some instances, unusually high truck loadings call for even greater thicknesses. Most pavements today include a subbase which is a stratum of material 4 to 20 inches thick between the pavement and natural subgrade. This subbase is a granular material sometimes treated for stabilization with either cement or asphalt, Design criteria, which account for soil support values, material strength characteristics and traffic, are used to determine the thickness of the pavement subbase.
Highway Development Record
The records of the development of roads and streets in the United States show a progressive increase in the total mileage and a continued conversion from the lower to higher surfacing and pavement types. The actual rate of development and change reflects not only the increase in funding for public highways to serve the rising traffic needs, but also the findings from research studies on surface materials, aggregates, reinforcements, mixes, construction and maintenance.
In 1904 there were about 2.35 million miles of public streets and highways. From 1910 to 1920 the total increased rapidly to about 3.2 million miles in all stages of improvement, ranging from primitive trails to highly improved urban thoroughfares. In 1920, 425,000 miles had some form of surfacing. Since 1950 the total mileage has increased consistently to the 1973 total of 3.8 million miles. The urban mileage has been expanding somewhat more rapidly than the rural mileage, particularly since 1950.
About 90 percent of the total road mileage was not surfaced in 1904. During the total mileage expansion in the teens, the nonsurfaced mileage actually increased, but the construction programs resulted in a gradual proportional decrease to about 80 percent in 1925. Since then the nonsurfaced mileage has decreased rapidly to about 20 percent in 1973.
Since about 1920 the total surfaced mileage has been increasing steadily to the 1973 total of 3.17 million miles. The gravel surfacing type has been dominant. Since about 1935 the rate of surfacing urban highways has been somewhat more rapid than that for the rural mileage.
The mileage of rural gravel-type surfaces (soil, slag, gravel and stone) increased regularly to about 1960. Since 1962 this type has been decreasing. It should be noted that the increase in gravel mileage up to 1960 was over and above the mileage that was upgraded to higher surface types. From 1935 to 1955 the rural gravel improved mileage was from 3 to 4 times that of the low bituminous mileage. The record on urban gravel mileage shows little change from 1941 to 1973, with a continuing total of 70,000 to 80,000 miles.
The low bituminous[N 1] rural mileage jumped substantially during the 1930's and increased steadily until the late 1960’s, when it leveled off. In the urban areas, the low bituminous mileage continually increased.
The high bituminous[N 2] rural mileage followed the upward trend of low bituminous type, with totals only about half to two-thirds that of the lower type. The decided increase about 1950 corresponds with the decrease in the low bituminous type. This type continued the upward trend the last few years, as distinct from the low bituminous mileage,
The mileage record of the concrete type differs considerably from the other types. For the rural mileage during the 1920’s and early 1930’s, the total exceeded that of both bituminous types. The existing rural concrete pavement mileage increased until 1935, leveled off for some 10 years, and since has decreased.
The mileage decrease for rural concrete pavements reflects the conversion of sections of that type of pavement to the high bituminous type when an overlay was placed as a maintenance step. The existing urban concrete mileage for the last 16 years shows no increase, the mileage being only half or less those of bituminous pavements. The overlay conversion effect doubtless applies here also.
Proportionally, there has been substantially higher type development of road and street mileage in the urban area. The percentages of the 1973 mileages for the various types were:
| Total | Rural | Urban | State Highway Systems | |
Total Existing Mileage (1,000’s)
|
3,807 | 3,176 | 631 | 764 |
| Percentage by Type: | ||||
| Nonsurfaced | 20 | 23 | 4 | 3 |
| Surfaced: | ||||
| Gravel | 34 | 38 | 11 | 8 |
| Low Bituminous | 24 | 21 | 40 | 33 |
| High Bituminous | 19 | 16 | 36 | 48 |
| Concrete | 3 | 2 | 9 | 8 |
| Total Surfaced | 80 | 77 | 96 | 97 |
REFERENCES
- ↑ Second Biennial Report of the Department of Public Roads Made to the Governor and General Assembly of Kentucky—Nov. 1, 1913 to Nov. 1, 1915 (The State Journal Co., Frankfort, Ky., 1915) pp. 35, 36.
- ↑ Fifth Report of the Illinois State Highway Department For the Years 1913, 1914, 1915, 1916 (Illinois State Journal Co., Springfield, Ill., 1917) p. 135.
- ↑ Sixteenth Annual Report of the Division of Highways—Jan. 1, 1933 to Dec. 31, 1933 (Illinois Department of Public Works and Buildings, Springfield, Ill., 1933) p. 10.
- ↑ Official Automobile Blue Book, various volumes for the year 1920 (Automobile Blue Book Publishing Co., New York).
- ↑ M. Duerksen, The Story of Abner Doble and His Magnificent Steam Cars, Cars & Parts, Jan. 1975, p. 102.
- ↑ Report of the State Highway Commission of Minnesota, 1915–1916,(Mar. 1, 1917) p. 18.
- ↑ Fourth Annual Report of the Division of Highways—Jul. 1, 1920 to Dec. 31, 1921 (Illinois Department of Public Works and Buildings, Springfield, Ill., 1922) p. 9.
- ↑ Eleventh Annual Report of the Division of Highways—Jan. 1, 1928 to Dec. 31, 1928 (Illinois Department of Public Works and Buildings, Springfield, Ill., 1929) p. 25.
- ↑ First Biennial Report of the California Highway Commission, 1917–1918 (State Printing Office, Sacramento, Calif., 1919) p. 51.
- ↑ Biennial Reports of the State Highway Commission, Commonwealth of Kentucky, various years from 1915 to 1921 (The State Journal Co., Frankfort, Ky.).
- ↑ Annual Reports of the Division of Highways, various years from 1915 to 1921 (Illinois Department of Public Works and Buildings, Springfield, Ill.).
- ↑ Id.
- ↑ Report of the State Road Commission of Maryland For the Years 1927, 1928, 1929 and 1930 (Baltimore, Md., Oct. 1, 1930) p. 85.
- ↑ Annual Reports of the Division of Highways, various years from 1921 to 1930 (Illinois Department of Public Works and Buildings, Springfield, Ill.).
- ↑ Report of the Commissioner of Highways of Minnesota For 1920 (Mar. 1, 1921) p. 4.
- ↑ Seventh Annual Report of the Division of Highways—Jan. 1, 1924 to Dec. 31, 1924 (Illinois Department of Public Works and Buildings, Springfield, Ill., 1925) p. 1.
- ↑ Seventeenth, Eighteenth and Nineteenth Annual Reports of the State Highway Commission For the Years 1924, 1925 and 1926 to the General Assembly of Maryland (Baltimore, Md., Jan. 1927) p. 12.
- ↑ Standards Approved by the American Association of State Highway Officials During the Year 1928, American Highways, Vol. 7, No. 4, Oct. 1928, p. 21.
- ↑ Aircraft Yearbook, 1927 (Aeronautical Chamber of Commerce of America, Inc., New York, 1927) pp. 96, 97.
- ↑ A Policy On Highway Classification (American Association of State Highway Officials, Washington, D.C., 1940) pp. 1, 2, 7, 8.
- ↑ A Policy On Sight Distance For Highways (American Association of State Highway Officials, Washington, D.C., 1940) pp. 4, 17.
- ↑ Id., p. 15.
- ↑ A Policy On Highway Types (Geometric) (American Association of State Highway Officials, Washington, D.C., 1940) pp. 65–69.
- ↑ A. Luedke & J. Harrison, Superelevation and Easement as Applied to Highway Curves, Public Roads, Vol. 3, No. 31, Nov. 1920, pp. 3–12.
- ↑ R. Moyer, Further Skidding Tests With Particular Reference to Curves, Proceedings, 14th Annual Meeting, Vol. 14 (Highway Research Board, Washington, D.C., 1934) p. 129.
- ↑ J. Barnett, Safe Side Friction Factors and Superelevation Design, Proceedings, 16th Annual Meeting, Vol. 16 (Highway Research Board, Washington, D.C., 1936) pp. 72, 73.
- ↑ A. Bruce, The Effect of Increased Speed of Vehicles on the Design of Highways, Public Roads, Vol. 10, No. 1, Mar. 1929, pp. 15, 16.
- ↑ H. Kelsey, The Future of Roadside Development In Massachusetts, (Address delivered at the annual meeting of the Massachusetts Forest and Park Association in Boston, Mass., Feb. 6, 1936) pp. 1, 2.
- ↑ Interregional Highways, H. Doc. 379, 78th Cong., 2d Sess., p. III.
- ↑ A Policy On Design Standards (American Association of State Highway Officials, Washington, D.C., 1941) p. 5.
- ↑ Traffic Devices: Historical Aspects Thereof (Institute of Traffic Engineers, Washington, D.C., 1971) p. 81.
- ↑ Id., p. 82.
- ↑ Id., p .103.
- ↑ C. Borth, Mankind On the Move (Automotive Safety Foundation, Washington, D.C., 1969) p. 204.
- ↑ ITE, supra, note 31, p. 24.
- ↑ Id., p. 35.
- ↑ Id., p. 43.
- ↑ Id., p. 35.
- ↑ Id., pp. 82–84.
- ↑ Report of First National Conference On Street and Highway Safety, Washington, D.C., Dec. 15–16, 1924, p. 21.
- ↑ Id., p. 26.
- ↑ A. Rose, Historic American Highways—Public Roads of the Past (American Association of State Highway Officials, Washington, D.C., 1953) pp. 52, 53.
- ↑ Id., p. 70.
- ↑ Fundamentals of Asphalt Paving (The Ohio Oil Company, 1949) pp. 18, 19.
- ↑ A. Rose, supra, note 42, p. 105.
- ↑ Id., p. 97.
- ↑ Third Annual Report of the Board of County Road Commissioners of Wayne County to the Board of Supervisors of Wayne County—From Sept. 16, 1908 to Sept. 30, 1909, p. 23.
- ↑ C. Older, Official Conclusions From the Bates Road, Public Works, Vol. 54, No. 1, Jan. 1923, p. 13.
- ↑ Final Report On Road Test One—Md.: Effect of Controlled Truck Axle Loadings On Concrete Pavement, Special Report 4 (Highway Research Board, Washington, D.C., 1952) pp. 7–11.
- ↑ The WASHO Road Test—Part 2, Test Data, Analyses, and Findings, Special Report 22 (Highway Research Board, Washington, D.C., 1955) pp. 4, 5.