Speed is only one of several factors affecting the occurrence of accidents. For example, much of the German Autobahn does not have a speed limit, yet the traffic fatality rate is lower than the fatality rate for US Interstate Highways.
The object of this paper was to review relevant accident data in order to establish a relationship between speed limits and accident rates, so as to facilitate a rational approach towards the setting of speed limits for the U.S. Interstate Highways.
Our results indicate that the main parameters effecting traffic accidents are: (a) Road design, (b) Driver alcohol consumption, (c) Seat belt usage. Speed limits were found to have minimal effect on the traffic accidents. The evidence also indicates that separate speed limits for day-time and night-time as well as for dry road and wet road would enhance safety much more significantly than the enforcement of speed limits that are set too low. A daytime dry road speed limit of 75 to 80 mph on rural Interstate Highways coupled with enforced seat belt usage would decrease the number of serious injuries resulting from traffic accidents.
Only 2 percent of the traffic accidents occur on rural Interstate Highways. Consequently, to decrease the number of traffic accidents, attention (funds, traffic law enforcement, etc.) should be focused on roads with speed limits 55 mph and lower. It is particularly important to improve the design of the rural roads (which have the highest fatality rates) presently posted with a 50-55 mph speed limit.