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https://www.sciencedaily.com/release...1212135021.htm

Speed limits set only five miles per hour below engineering recommendations produce a statistically significant decrease in total, fatal and injury crashes, and property-damage-only crashes, according to a group of Penn State researchers.

"If (however) you lower the speed limit by 10, 15, 25 miles per hour, or more, drivers stop paying attention," said Vikash Gayah, assistant professor of civil engineering. "We found there was an increase in fatal and injury crashes at locations with posted speed limits set 10 miles per hour or more below engineering recommendations."

Speed limits are normally set based on results from engineering studies that collect free-flow traffic data and then select an appropriate speed using a statistical model. However, factors such as school zones, citizen or political pressure, and perceived safety issues contribute to the fairly common practice of lowering speed limits below engineering guidelines, the researchers report in Accident Analysis and Prevention.