report was adopted by the Third National Conference on Street and Highway Safety in 1930. It was in effect a manual of the best practices of the time, including much of the practices embodied in AASHO’s manual. Recognizing that there could not be different standard practices for signing in rural and urban areas, AASHO and the National Conference organized a Joint Committee on Uniform Traffic Control Devices[N 1] in 1931, which in 1935 brought out a new manual for national use. This manual, periodically revised to keep pace with traffic developments, has been a powerful force for uniformity and traffic safety in the United States.
Until 1924, there was no national agreement as to uniform road signs; thus, from region to region there were an infinite variety.
The Consequences of Speed
In the early days of the automobile, legal speed limits were set far below the speeds of which most motor vehicles were capable.[N 2] Horsedrawn vehicles were numerous on the highways, and teams might bolt if they were approached or passed at high speed. High speeds aggravated the dust nuisance and accelerated the destruction of macadam surfaces. Roads were narrow—generally less than 16 feet wide—and often flanked by deep ditches so that other vehicles could be passed safely only at low speed. Finally, the vehicles, and especially their tires, were of uncertain reliability; blowouts and loss of steering control were fairly frequent, and these could be disastrous at speeds greater than 25 miles per hour.
All of these factors changed as motor vehicle ownership increased. Animal-drawn traffic decreased and became numerically and politically unimportant. Bituminization solved the dusting problem, and vehicles and tires became more reliable. Drivers, feeling safer and more comfortable, increased their speeds and were able to exert enough political pressure to have speed limits raised also.
With heavier traffic and higher speeds, it became dangerous to drive in the middle of the road, and the States began painting centerlines on the pavements to channelize traffic in lanes.[N 3] At 40 miles per hour,
- ↑ The Joint Committee now consists of members from AASHO, the Institute of Traffic Engineers, The National Committee on Uniform Traffic Laws and Ordinances, The National Association of Counties and The National League of Cities.
- ↑ In 1918 the legal limit in South Carolina was 15 miles per hour and five States had limits of 20 m.p.h. The limit in most States was 25 to 30 m.p.h., but in Kansas 40 m.p.h. was permissible. Eight States had no speed limit. In 1928, only Massachusetts still had a 20-mile per hour limit, and in 32 States the limits were 35 to 40 miles per hour. Four permitted 45-mile per hour speeds.[1]
- ↑ In 1920 Marquette County, Michigan, began painting white centerlines on curves. These lines were hand-painted with whitewash and lasted only a month on the road, but were effective for keeping drivers on their own side of the road.[2] About 1925 it became general practice to build concrete highways with a center joint to control longitudinal cracking. This joint became a visible line separating the two lanes of traffic and served the same purpose as a painted stripe.
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