Why does the MUTCD allow the signal warrant volumes to be reduced to 70 percent of the normal values in certain conditions?
The KLD NCHRP study report from 1976 indicates that the 70% factor emerged in the late 50’s (when the 1961 MUTCD was being written) as a “compromise” between rural and urban traffic engineers then on the NCUTCD. In prior editions of the MUTCD (1948 and earlier) there had been separate urban and rural warrant numbers, and the NCUTCD was trying to revise and consolidate them into a single set of numbers applicable for all conditions. Rural interests (predominantly the State highway departments) favored lower warrant numbers so that they could justify signals at rural intersections and in the downtowns of the small rural communities, both of which typically exhibited lower volumes than in the cities. The urban interests felt that the higher (100%) numbers should be retained because with lower warrant values the number of signals they would have to install in the rapidly developing cities would greatly increase. Citing conversations with many “old-timer” NCUTCD members at that time, the KL
