Neither raising nor lowering the speed limit had much effect on vehicle speeds. The mean speeds and the 85th percentile speeds did not change more than 1 or 2 mi/h (1.6 or 3.2 km/h), even for speed limit changes based on the amount the posted speed limit was altered. The percent compliance with the posted speed limits improved when the speed limits were raised. When the speed limits were lowered, the compliance decreased. Lowering the speed limit below the 85th percentile or raising the limit to the 85th percentile speed also had little effect on drivers' speeds.which merely confirms numerous previous studies. Interestingly, over two decades apart, the Ohio Dept. of Transportation reported virtually identical 85th percentile speeds on Ohio rural interstates:
- 72-mph according to 1995 speed data gathered by ODOT (reported by the Patrol's "study")
- 72-mph according to 1973 speed data gathered by ODOT (before the oil-price-crisis and federal intervention)
Michigan recently raised their limit to 70-mph, after a 3-month test on selected freeways. Governor Engler's press release {link to Michigan Governor's website} quoted State Transportation Director Robert Welke as saying "One good test is worth a thousand expert opinions. The test proved this old adage true, as the data shows no significant increase in freeway speeds."
Many people, desperate for simple solutions, treat the effectiveness of speed limits and law enforcement as an unquestionable Article of Faith (which is hardly a basis for sound public policy.) Occasionally glimmers of truth spring to the public eye: consider an article in the Washington Post, (National Weekly Edition, p. 38, September 9-15, 1991) entitled "Trafficking in the Unpredictable," which noted:
When a team of government experts sat down more than 15 years ago to predict whether U.S. traffic deaths would rise or fall over the coming decade, the answer seemed obvious. Population increases would put more people on the roads. Average highway speeds were going up, as was per capita alcohol consumption. Although there were more safety features in cars, cars were getting smaller and tighter. On the balance, the experts predicted that the number of highway fatalities were soar from 40,000 in 1975 to 72,300 in 1985.They were wrong. Fatalities declined. The problem wasn't the team's assumptions. Estimates about population, speed and car safety were uncannily accurate. The problem was that every one of the factors thought to be important turned out not to have much effect.
For fun, check out the (almost real-time) speeds on San Diego freeways. {link to Caltrans site}
Back to the Ohio NMA rootpage.
Updated May 20, 1997