IIHS: Lower urban speed limits mean less speeding

August 28, 2018

At the annual Governors Highway Safety Association meeting Tuesday in Georgia, the IIHS released the results of a study showing Boston's recently lowered speed limits reduced the number of cars unsafely speeding.

The insurance industry-funded group conducted its study in Boston, where speed limits were lowered in certain areas from 30 to 25 mph, and in Providence, Rhode Island, which already had 25 mph urban speed limits.

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In Boston, the IIHS found that the odds of a vehicle exceeding 35 mph on a 25 mph street fell by 29.3 percent and that the odds of a vehicle exceeding 30 mph dropped by 8.5 percent with the slower speed limits.

In both cities, the IIHS deployed traffic flow counters in 5-hour blocks from 10 a.m. to 3 p.m. over two weekdays before and after Boston's speed limit change. The IIHS used 50 data collection sites in each city, which it said in its report were selected because of similar geography and a lack of posted speed limit signs.

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The IIHS champions the 25 mph speed limit because the 5 mph reduction results in a considerably lower risk of pedestrian fatalities. Citing AAA Foundation for Traffic Safety data the IIHS said in a statement that, "a pedestrian struck by a vehicle at 25 mph has a 25 percent risk of sustaining a serious or fatal injury, and the risk jumps to 50 percent at 33 mph and 75 percent at 41 mph."

The speed limit reduction in Boston was made possible after the Massachusetts Legislature in 2016 amended state law to allow cities and towns to lower speed limits from 30 to 25 mph in high-density areas.

Although the IIHS study did not examine how the lower speed limits affected the risk of crashes in Boston, the nonprofit hinted that a more detailed study could come in the future.

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