Drivers were less likely to drive at excessive speeds after the speed limit was lowered for Boston streets, according to an analysis by the Insurance Institute for Highway Safety.

After Massachusetts Governor Charlie Baker signed legislation granting cities and towns in the state the authority to lower their default speed limit, Boston reduced its speed limit from 30 miles per hour to 25 miles per hour. The switch, which was made as part of its Vision Zero goal to eliminate serious or fatal vehicle crashes by 2030, went into effect in January 2017. Residents were informed of the change through an advertising campaign as well as both social media and traditional media outlets.

To determine the effect of the speed limit reduction, IIHS researchers compared vehicle speeds on arterial roads, collectors, and local roads from before and after the change. Boston vehicle speeds were also compared to similar roads in Providence, R.I., which has not lowered its speed limit.

None of the roads included in the study had a posted speed limit. However, researchers found that Bostonians were less likely to drive at excessive speeds after the speed limit reduction and accompanying public information campaign.

The estimated odds of a vehicle exceeding 35 miles per hour fell by 29.3 percent after the speed limit was dropped to 25 miles per hour. The odds of a vehicle traveling faster than 30 miles per hour fell by 8.5 percent, while the odds of a vehicle exceeding the 25 mile-per-hour speed limit itself fell by 2.9 percent. Average vehicle speeds fell by 0.3 percent.

"Speeding occurs on roads of all types, not just highways and freeways," said David Harkey, president of IIHS. "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."

IIHS says lower speed limits are particularly beneficial for more vulnerable road users such as pedestrians and bicyclists, since these people have a greater chance of surviving if they are struck by a slower moving vehicle. According to the AAA Foundation for Traffic Safety, a pedestrian has a 25 percent chance of sustaining a serious or fatal injury when struck by a vehicle traveling at 25 miles per hour. The risk increases to 50 percent at 33 miles per hour and 75 percent at 41 miles per hour.

Speed limits are commonly set based on the 85th percentile speed of free-flowing traffic on a road, or the speed that 85 percent of vehicles are traveling at or below when traffic is not congested. However, IIHS says this approach can ignore important factors related to the road's use and design.

"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," said Harkey. "Our new study shows that safety benefits can be gained when speed limits take into account all road users in an urban environment."

IIHS has previously criticized speed limit increases on highways, saying this change does not set a limit that more closely hews to actual traffic speeds but rather encourages drivers to travel even faster, increasing the risk of serious crashes. A 2016 study concluded that increases to speed limits in 41 states between 1993 and 2013 were associated with at least 33,000 fatalities that could have been avoided with lower speed limits.

The study on Boston's speed limit did not look into what effect the speed limit reduction may have had on crashes. However, Harkey said this factor will be analyzed in future research.