The 55 mph limits and front-to-rear collisions involving autos and large trucks

Abstract

The effects of the establishment of 55 mph limits on front-to-rear crashes involving automobiles and trucks were examined. Since the establishment of 55 mph limits resulted in a reduction in the difference between the reported average speed of automobiles and large trucks, it provided an opportunity to examine the effects of speed differences on the. frequency of crash involvement of these vehicles. Principal results of the study were as follows: In 1974, the year of the introduction of 55 mph speed limits, a substantial reduction in the number of front-to-rear crashes involving an automobile and tractor trailer on higher speed roads occurred in the states whose experience was examined. A substantial decline in the number of front-to-rear crashes involving an auto and single body truck on higher speed roads also occurred in 1974 in these states. The decline in the number of front-to-rear crashes involving an automobile and tractor trailer on higher speed roads was primarily the result of a major decline in the number of crashes in which an auto struck a tractor trailer in the rear. The number of crashes in which a tractor trailer struck an auto in the rear declined by a much smaller percentage. The decline in the number of front-to-rear crashes involving an automobile and single body truck on higher speed roads resulted from comparable decreases in the number of crashes in which an auto struck a single body truck in the rear and those in which a single body truck struck an auto in the rear. Prior to the establishment of 55 mph limits, tractor trailers struck automobiles in the rear in more than half of the front-to-rear crashes involving these vehicles on both higher and lower speed roads. Because the major decline in the number of crashes in which autos struck tractor trailers in the rear following the introduction of the new limits was not matched by as large a decline in the number of crashes in which tractor trailers struck autos in the rear, there was a significant increase in the proportion of front-to-rear crashes involving an automobile and tractor trailer in which the tractor trailer struck the automobile in the rear on higher speed roads. Prior to the establishment of 55 mph limits, single body trucks struck automobiles in the rear in more than half of the front-to-rear crashes involving these vehicles on both higher and lower speed roads. However, single body trucks struck autos in the rear in a lower proportion of their front-to-rear crashes with autos than did tractor trailers. The proportion of front-to-rear crashes involving an automobile and single body truck in which an auto was struck in the rear by a single body truck was not significantly affected by the establishment of 55 mph limits.