Hit The Gas And Save Lives?

The paradox of higher speed limits

Does speed kill? As far as motor vehicles are concerned, the answer seems to be yes. When speed limits were cut to 55 mph in the mid-1970s, for example, traffic fatalities dropped sharply. And deaths on rural interstate highways rose when most states raised speed limits to 65 mph in 1987.

More recently, several states that raised limits after federal restrictions ended in 1995 have reported a surge in traffic deaths. In Texas, which adopted a 70-mph limit on most state-maintained roads, fatalities jumped 18% last year.

These statistics, however, tell only part of the story. As it happens, national traffic-fatality rates--deaths per million vehicle miles--have been falling continuously for 70 years. And despite recent increases in a few states, the actual number of fatalities nationwide has been declining on a year-over-year basis since mid-1996 (chart).

Moreover, Charles Lave of the University of California at Irvine observes that Congress acted to raise the speed limit in 1987 partly because widespread evasion of the law was eroding its effectiveness. "The growing variance in speed between the vehicles of lawbreakers and law-abiders was causing more and more accidents," he says. Indeed, in a recent study, Lave and colleague Patrick Elias find that the 1987 speed-limit increase actually lowered fatality rates significantly.

The two economists claim that federal sanctions attached to the 55-mph limit had forced state highway patrols to shift limited resources to monitoring interstate highways, which were already the nation's safest roads, accounting for only 9% of fatalities. The low limit also prompted many disgruntled motorists to shift to other, less-safe--and now less-policed--roads. When it was raised to 65 mph, however, traffic shifted back to the far safer interstates, and police resources were more effectively allocated--lowering overall fatalities.

To test this thesis, the study compared states that adopted the 65-mph speed limit in 1987 with those that kept the 55-mph limit. In the first group, it found that traffic on the interstates in 1987 grew 1.75 times faster than overall traffic and 1.62 times faster than interstate traffic in the 55-mph group.

At the same time, though highway deaths in the states with higher speed limits rose, their overall fatality rate (reflecting driving on all roads) fell by 4.68% in 1987 and an additional 1.55% in 1988. By contrast, fatality rates in the 55-mph states showed no change in 1987 and fell by 2.55% in 1988. The two researchers conclude that the speed-limit increase cut fatality rates by 3.62%.

As for the nation's latest exposure to higher speed limits, Lave says fatalities increased in Texas because it chose to raise limits on all highways, even dangerous two-lane rural roads. "But the national results are quite different: Fatalities have actually declined," he says.