Slate’s explainer: Do higher speed limits cause more accidents?

Posted June 14, 2012, at 8:01 p.m.
Last modified June 14, 2012, at 9 p.m.

The Texas Department of Transportation is debating whether to increase the speed limit along a 41-mile length of road between San Antonio and Austin to 85 mph. If they agree to the boost, this stretch of State Highway 130 would host the highest speed limit in the country and the second highest in the world. Do higher speed limits cause more car accidents?

No, but they do cause more severe ones. Accidents that occur at high speeds are more often fatal, since high-velocity objects collide with greater force. Overwhelmingly, studies show that freeway deaths increase with freeway speed limits, although it’s hard to know how much of that upturn stems from the greater volume of overall traffic that’s drawn in by looser speed regulations.

What a raised speed limit probably won’t do is inflate the number of collisions. It’s often assumed that higher caps will make roadways more dangerous, because motorists will exceed whatever ceiling is in place. Likewise, people think lower speed limits will promote safety by tamping down velocity. In fact, studies show that drivers rarely overshoot their speed comfort zones, even with legal encouragement — and that the perception of hazard on freeways with high speed limits can actually boost safety by heightening drivers’ caution.

Two recent cases helped refute the notion that relaxed speed limits lead to more accidents. After New York raised the cap on its highway traffic to 65 mph in 1995, the state’s total crash rate dropped by 4 percent. And in 2000, the Automobile Club of Southern California determined that higher speed limits in that state did not increase the rate of statewide accidents over a period of five years. A study of Iowa’s 1996 increase of the state speed limit from 55 to 65 mph concluded that this speed hike caused a jump in traffic accidents. In a lengthy literature review of traffic engineering studies, however, it stands alone in its conclusion that higher speed limits cause more crashes.

The key to road safety is uniform traffic flow: Accidents occur when cars move at divergent rates. In the 1950s, pioneering traffic researcher David Solomon determined that cars are more likely to crash when they are travelling above or below the median traffic speed, regardless of what that speed is. Highway authorities now typically set speed limits by establishing the “85th percentile speed” — that is, according to the Texas Department of Transportation, “a speed at or below which 85 percent of people drive at any given location under good weather and visibility conditions.” Empirical data allows state highway authorities to account for the true-to-life road conditions, rather than imposing an artificial cap on traffic flow.

Getting people to abide by new speed limits isn’t easy. A 1996 study by the National Motorists Association suggests that drivers will adjust their velocities only slightly in response to new legal strictures. In a study of 22 states where speed limits were either raised or lowered by five, 10, 15 or 20 mph, researchers found that cars’ average velocities did change, but by less than two mph. Rather than the tempo of travel, shifting road regulations altered the rate of compliance: Violations of the speed limit increased when limits were lowered, and decreased when limits went up.

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