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Traffic Study Yields Interesting Results

Last updated on Wednesday, June 25, 2008

(WEST LAFAYETTE) - It’s been three years since Indiana raised the speed limit to 70 miles an hour on parts of I-65. A Purdue study finds the likelihood of serious accidents on the road hasn’t changed.

Several studies found cutting speed limits to 55 in 1974 reduced both crashes and highway deaths. Civil Engineering Professor Fred Mannering says cars have changed so much since then, with anti-lock brakes, air bags, and increased seat belt use, that his study found no significant change when the speed limits went back up.

Mannering says the higher limit also appears to keep drivers moving at about the same speed. Instead of having one driver doing 50 and another going 80, it's more likely that drivers will cluster within a few miles per hour of the 70-miles-an-hour limit. That improves traffic flow and makes roads safer.

Crashes on state highways in 2006, the year after the limit changed, were actually down 10% from 2004, the last full year with the lower limit. Mannering says 2004 featured an unusually snowy winter, which caused unsafe driving conditions. His study filtered out weather and dozens of other variables to arrive at accident probabilities for each year.

Mannering says there's no way to tell without trying whether an even higher speed limit would be safe. Other states have limits as high as 80, but driving behavior, weather patterns and terrain may be different.

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