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IIHS News | August 28, 2018Subscribe

City drivers slow down for lower speed limit in Boston

ARLINGTON, Va. — Lowering the speed limit by 5 mph on city streets can improve safety for motorists, pedestrians and bicyclists alike by reducing speeding, new research by the Insurance Institute for Highway Safety indicates. Released today at the Governors Highway Safety Association (GHSA) 2018 Annual Meeting in Atlanta, Ga., the study bolsters Vision Zero efforts to reduce traffic-related deaths among all road users.

"Speeding occurs on roads of all types, not just highways and freeways," says IIHS President David Harkey. "Even on lower speed roads, speeding can have deadly consequences, especially for pedestrians and bicyclists. Some cities are lowering speed limits to reduce the risks for these vulnerable road users, who are increasingly dying in crashes."

The IIHS study focused on Boston, which lowered the default speed limit on city streets from 30 mph to 25 mph beginning January 9, 2017, and publicized the change via advertisements, social media and traditional media outlets. The city's move came after the Massachusetts legislature in 2016 amended state law to allow cities and towns to lower speed limits from 30 mph to 25 mph on municipal roads in densely populated areas or business districts. Unless otherwise posted, the speed limit on all City of Boston roadways is 25 mph.

IIHS researchers looked at vehicle speeds in Boston before and after the lower limit took effect and compared them to control sites in Providence, Rhode Island, where the speed limit remained the same. The study sites in both Boston and Providence included arterials, collectors and local roads. All sites were similar in that they had no more than one lane per direction and were located away from intersections on relatively flat, straight road segments and at least a half-mile away from any school or speed feedback sign. There was no posted speed limit sign at any of the sites.


After Boston lowered the default speed limit to 25 mph,
the estimated odds of a vehicle

After Boston lowered the default speed limit to 25 mph, the estimated odds of a vehicle

exceeding 35 mph

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fell 29.3%

exceeding 30 mph

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fell 8.5%

exceeding 25 mph

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fell 2.9%


Researchers evaluated changes in the odds of vehicles exceeding 25 mph, 30 mph and 35 mph associated with the new speed limit.

"We found significant reductions in the odds of vehicles in Boston exceeding 25 mph, 30 mph and 35 mph associated with the reduced speed limit, and the decline was biggest for the odds of vehicles exceeding 35 mph," Harkey says.

There was a 29.3 percent decline in the odds of speeding for vehicles traveling faster than 35 mph. The odds of speeding fell by 8.5 percent for vehicles going faster than 30 mph and 2.9 percent for vehicles exceeding 25 mph.

The study didn't examine how lowering the speed limit affected crashes in Boston. That's a next step, Harkey says.

High travel speeds increase the risk of crashing and the risk of injuries when a crash occurs. Speeding, defined on police crash reports as exceeding the posted speed limit, driving too fast for conditions, or racing has been a factor in more than a quarter of U.S. crash deaths for more than 30 years.

In 2016, the percentage of U.S. crash deaths related to speeding was higher on roads with 35 mph or lower speed limits than on roads with higher speed limits (33 percent vs. 26 percent).

As part of Vision Zero programs, Boston, New York, Seattle and other cities have lowered speed limits as one way to make streets safer for pedestrians and bicyclists.

Pedestrian deaths in the U.S. have climbed 46 percent since reaching a low point in 2009, increasing at a higher rate in urban areas than less-populated ones. More than 70 percent of pedestrian deaths during 2009–16 occurred in urban areas, IIHS research shows.

The faster a car is moving, the less time the driver has to see a pedestrian and slow down or stop and the higher the injury risk for the pedestrian. Even small increases in vehicle speed can have fatal results. 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, research by the AAA Foundation for Traffic Safety shows.

Outside of urban areas, speed limits are on the rise. Forty-one states have speed limits of 70 mph or higher on some portion of their roads. Raising speed limits on interstates and freeways increases speeds and fatality rates. A 2016 IIHS study showed that increases in speed limits from 1993 to 2013 in 41 states have cost 33,000 lives in the U.S.

"The new IIHS study should encourage officials to reexamine current speed limits to find opportunities to more effectively lower overall speeds and reduce speeding-related traffic deaths," says GHSA Executive Director Jonathan Adkins. "Whatever the speed limit, enforcement is key and especially critical on higher speed roadways."

State and local agencies routinely set speed limits based on the observed operating speeds on road segments, specifically the 85th percentile speed of free-flowing traffic. In Boston, the 85th percentile speed remained unchanged at 31 mph before and after the reduced default speed limit took effect.

"Using only the 85th percentile speed to set speed limits on roads often ignores the design and function of the roadway," Harkey says. "Crash statistics, road use by pedestrians and bicyclists, presence of driveways and intersections, and curvature of the road are all factors to consider when setting speed limits. Our new study shows that safety benefits can be gained when speed limits take into account all road users in an urban environment."

To address the problem of speeding, IIHS and GHSA are partnering on a speed forum to be held in April 2019 at the IIHS-HLDI Vehicle Research Center in Ruckersville, Va. The forum will bring together policymakers, insurers, highway safety advocates and law enforcement officials to discuss the latest research and countermeasures.

"We need multiple partners working together to accelerate the implementation of countermeasures that we know work, such as automated enforcement, and to develop new strategies that can address this enduring road safety problem," Harkey says.

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