Raising the speed limit from 65 to 70 on Interstate 65 in Indiana has not increased the probability of fatalities or severe injuries, a new study finds.
The research adds to a long-running controversy over the relationship between highway speeds and accidents.
"These findings are important because the influence of speed limits on roadway safety has been a subject of continuous debate in the state of Indiana and nationwide," said researcher Fred Mannering, a professor of civil engineering at Purdue University.
A nationwide study in 2005 reached similar conclusions. But researchers say the results could reflect, at least in part, safer cars and safer roads.
Each year, more than 40,000 Americans are killed and nearly 3 million injured in some 6.3 million traffic accidents. The odds of dying in a traffic accident are about 1-in-100 for a U.S. resident. And motor vehicle crashes are the leading cause of death for those ages 2 through 33.
"The safety of raising the speed limit has been a matter of considerable concern in Indiana since the state raised its speed limits on rural interstates and selected multilane highways on July 1, 2005," Mannering said. "Everybody expects that when you increase the speed limit, injuries and the severity of injuries are going to increase, but that hasn't happened on the interstate highway system in Indiana."
The findings will be detailed in Transportation Research Record and were presented earlier this year at a meeting of the Transportation Research Board of the National Academies.
The federal government mandated a 55 mph limit on interstate highways in 1974 to save gas. It was changed to 65 mph in the 1980s then repealed in 1995. Now states set the speed limits. Indiana was the 30th state to raise interstate speed limits up to 70 mph on rural interstates.
Studies on the topic have generated results all over the ballpark.
"For example, one study found that a speed limit increase from 55 to 65 resulted in roughly a 3 percent increase in the accident rate and a 24 percent increase in the probability of a fatality once an accident occurred," Mannering said. "But then other studies have contended that legislation-enabled speed-limit increases have actually saved lives. One study argued that increasing from 55 to 65 saved lives because of shifts in law enforcement resources, the ability of higher speed limit interstates to attract riskier drivers away from inherently more dangerous non-interstate highways and reducing how often drivers speed up and slow down."
Findings by other researchers suggest even higher speed limits on interstate highways might also result in no rise in the probability of severe injuries.
"If going from 65 to 70 doesn't have a significant effect on the severity of accidents, you have to ask yourself, what about 70 to 75?" Mannering said. "At what point does it begin to impact safety?"
However, Mannering said any future speed-limit increases should be considered on a case-by-base basis.
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