Page:America's Highways 1776–1976.djvu/137

 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 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.

In the early days of the automobile, legal speed limits were set far below the speeds of which most motor vehicles were capable. 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. At 40 miles per hour,

131