When Congress imposed the federal 55 mph speed limit in 1974, it did so to save lives. The argument that ''speed kills'' was repeated against the return of speed-limit-setting power to the states in late 1995. The Department of Transportation predicted that as many as 6,400 more people would die each year as a result.
Well, the results are in for 1996, the first year under the higher speed limits. Even though limits were raised to as high as 75 mph in some states the 41,907 total highway deaths in 1996 were a statistically insignificant 109 more than in 1995. The 1.7 deaths per billion miles driven did not change from 1995 (it has remained steady since 1992, at the lowest rate in history). Traffic safety has improved in the 36 states that set higher limits. California and Mississippi, which raised top speed limits to 70, saw 4 percent and 21 percent drops in fatalities (to the lowest level since 1961 in California). Even Montana, which abolished daytime speed limits entirely, saw highway fatalities fall from 215 to 200.Why was the official analysis of increased speed limits so far off? The same reason so many government programs fail - the law of unintended consequences (a government action intended to achieve one goal will have other, unintended consequences). Changing speed limits changes at least three other crucial highway safety determinants, but the exclusive focus on speed led policy makers to ignore these effects.
First, enforcing highway speed limits that are far slower than most drivers want to go absorbs a large share of highway patrol time and resources. When higher speed limits are allowed on those highways, some of those efforts can be diverted to areas with a greater impact on safety, such as increased patrolling of more dangerous roads (major highways were designed for safety at far more than 55 mph when cars were far less safe, but the same is not true of other roads) or programs to reduce drunk driving (involved in just under half of all traffic deaths nationwide).
Even if more speed on the interstates increased fatalities, as long as more lives were saved by the same resources devoted to other roads or other programs, higher speed limits would actually save lives.
Second, the faster you go, the less time you spend on the road to cover a given distance. This is especially important in the West, where drivers make far more long trips, because the last few hours on such trips are the most dangerous. By letting drivers avoid some of those last hours, higher speed limits on roads built for such speeds can save lives as well.
Third, differences in speed between vehicles, not the speed itself, is the source of many highway accidents. By raising the speed limit, the variance in speed between seemingly supersonic ''freeway flyers'' and more law-abiding citizens who stay closer to the legal limit can be reduced, diminishing the risk of accidents between them.
Setting nationwide speed limits in 1974 to save lives seemed like a relatively straightforward proposition, compared with the far more complex social issues the federal government routinely involves itself in.
But as the DOT data show, even for such a simple issue, the law of unintended consequences turns that policy into a resounding failure.
This begs the question, of course, whether the government, failing, as it has, in the simple task of regulating the speed on the highways, can tackle far more complex problems.