When this approach was tested in a number of States in which the needed investment by years to achieve the accepted level of service had been calculated by accumulating the route-by-route needs, the agreement in the total figures was indeed remarkable. The investment life approach could not replace the detailed survey, for it could not be directed toward particular routes or sections necessary to program construction expenditures. But it gave a check on the detailed engineering method and also provided a simple, easily understood means of keeping abreast of progress year by year as the highway program proceeded.
In somewhat the reverse of the highway needs situation, traffic volume forecasting was usually approached on an areawide basis. The classic procedure was to estimate population, motor vehicle ownership per capita, and annual miles per vehicle over the years and from these estimates calculate the total vehicle miles of travel to be expected. In the “depression psychology” still persisting, estimates in all three elements generally were too pessimistic, with the multiplication of all three producing far too low estimates of traffic on which to base future highway needs calculations. In one case of a highway needs study, the level estimated for 10 years ahead had been reached before the report was printed. In exploring this situation to try to find a more realistic basis for forecasting, it was discovered that, for the decade of the thirties, traffic had been growing at almost the same rate as the national economy and that, once the aberrations of the war were behind us, parallel growth was resumed. On the strength of that finding, in 1950 it was suggested by E. H. Holmes, Chief of the Highway Transport Branch of BPR, that this relationship was entirely logical and that in planning ahead it would be unwise to anticipate a growth rate below that of the last two decades—4 percent per year compounded. (This rate of growth would see traffic double in 17½ years; it did double in 16½.) This forecast was greeted with skepticism, to put it mildly, perhaps not because of doubts as to the relationship, but more by doubts that that rate of economic growth could continue. It was not until 1954 that the Bureau of Public Roads formally accepted the soundness of this relationship and raised its sights accordingly. Actually over the past two decades, the gross national product has been growing at about 3 percent annually while traffic has continued its 4 percent rate, presumably as our economy shifts from an industrial to a service oriented society and with a generally higher economic level.
But even the more realistic approach to traffic growth forecasting on an areawide basis was of little help in estimating probable volumes on specific routes. With the aid of selected States, origin and destination studies were carried on before and after major route improvements were made, and studies were made comparing parallel routes, such as toll roads and parallel existing routes. These studies disclosed that the traffic on the new routes could be segregated into four groups: trips “diverted” from other routes by the higher level of service; trips “generated” simply because the new routes offered a convenience not previously available; “development” trips resulting from new development along the routes; and finally, some trips simply resulting from “general traffic growth.” While subsequent research has refined this approach, estimates using these four general divisions of composition of the traffic stream were generally the basis on which the States estimated needs, route by route, required for the important studies to be conducted in 1954 and 1955.
These were examples of developing and improving planning techniques through research. During this period, great advances were made in processing, analyzing and interpreting data collected in the urban travel habit studies. As noted earlier, with the data processing equipment then available, the survey method was validated in the early studies. Travel could be accurately reproduced through home interviews. But to project travel into the future, the relationships between travel and land use had to be established, and as also noted, it took the computer and the introduction of new disciplines into the process to do it.
What emerged from this effort were a series of relationships, called models in computer terminology, that first showed the trips that might be expected from a zone of known land use to other zones of known land uses, as determined from the actual travel in the area. Then with the number of trips from each zone to each other zone calculated, another series of models to permit the “assigning” of the trips to the links of an existing or proposed network of routes, either highway or transit, was developed. The first series of models employed economic and social factors available by zones from census records, such as occupation, income level, and number in the family and their ages. The second included physical factors such as the purpose, time and distance of trips by various routings, costs of parking if that were involved, cost of travel by transit, and other factors to predict the mode and probable route of travel. The intensive work in this area led to a computer program “package” of models that permitted the estimation of trips and their routing with satisfying (to some, surprising) accuracy. In application of these models to estimate future travel and transportation facility needs, future land use had to be estimated. Research showed that computer simulation could also be used to predict the probable use of land, basing the models largely on theory and testing them against actual changes in land use as the urban areas expanded. In that success it may be said that the quantitative approach had been brought to urban planning by the transportation planners. The use of these models, in combination, can show what transportation facilities are required to provide a desired level of service for a given land use; they will also show what density of land use may be allowed if a desired level of transportation service on a given transportation network is to be maintained. These models, refined as they have been over the years, can appraise the effect of land use on transportation, as well as the reverse, the effect of transportation on land use. They can show how to achieve balance between transportation and other development.
