Managing Speed
Speed limits for regulating driving behavior date to 1901, but they remain controversial. The imposition of a nationwide 55-mph speed limit following the oil embargo of 1973, the allowance for states to raise limits to 65 mph on rural Interstate highways in 1987, and the allowance for states to increase limits on other major highways in 1995 were all hotly debated and the subject of numerous special studies.
Speed limits for regulating driving behavior date to 1901, but they remain controversial. The imposition of a nationwide 55-mph speed limit following the oil embargo of 1973, the allowance for states to raise limits to 65 mph on rural Interstate highways in 1987, and the allowance for states to increase limits on other major highways in 1995 were all hotly debated and the subject of numerous special studies.
A 1984 TRB study, requested by Congress, evaluated all the available research on the effects of the 55-mph national maximum speed limit (Special Report 204: 55: A Decade of Experience; TRB 1984). The committee concluded that 10 years after the reduction in maximum limits, 2,000 to 4,000 lives were being saved each year as a result of slower travel speeds relative to the period before the 55-mph national limit was imposed. These savings were apparently due to the reduction in average speeds and the reduced variability of speed on the highways. Reduced average motorist speed should reduce the risk of injury when a crash occurs because it typically results in a lower impact speed, whereas reduced variability in speeds among motorists should reduce the frequency of vehicle-to-vehicle collisions.
With each passing year, however, motorists’ compliance with the 55-mph limit was eroding. Even by 1984, the majority of motorists were exceeding the limit. Congress responded by allowing the national maximum limit to increase to 65 mph and subsequently abolished the national limit altogether.
In 1998 a TRB committee revisited this issue (Special Report 254: Managing Speed: Review of Current Practice for Setting and Enforcing Speed Limits; TRB 1998). Its report provides a retrospective on the experience with national maximum limits and suggestions for future policy. The committee’s analysis of the studies of speed limits supports the general finding that higher speed limits increase risk, but the committee also took note of the wide variety of local conditions and the statistical problems involved in isolating speed from other factors. Both this and the 1984 committee agreed that motorists’ compliance is critical and that enforcement is difficult.
In general, state and local officials are probably in the best position to judge appropriate maximum limits for highway systems in their locales, but they should also allow for speed zones where limits would be established to take account of both design issues and motorist behavior. Legislated system limits should be based on the design speed, vehicle operating speeds, safety experience on the route, and enforcement experience. The established approach of basing speed limits in speed zones solely on the 85th percentile of free-flow speed is not always appropriate; an expert-system approach would take into account a range of important factors.
Enforcement of speed limits has always been a challenge and is made more so when most motorists believe a limit to be unreasonable. Even when the majority comply, it is difficult to deter the highest-speed motorists, who put others at risk. Deployment of enough officers to catch or deter high-speed drivers can impose more expense than many jurisdictions are willing or able to bear. Automated technologies, such as photo-radar or intelligent systems, should be tested for both efficacy and public acceptability.