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Lowering the speed limit from 30 to 25 mph in Boston: effects on vehicle speeds

Hu, Wen; Cicchino, Jessica B.
Insurance Institute for Highway Safety
August 2018

Introduction: Effective January 9, 2017, the default speed limit on City of Boston streets was reduced from 30 mph to 25 mph. This study evaluated the effects of the speed limit reduction on speeds in Boston.
Method: Vehicle speeds were collected at sites in Boston where the speed limit was lowered, and at control sites in Providence, Rhode Island, where the speed limit remained unchanged, before and after the speed limit change in Boston. A log-linear regression model estimated the change in vehicle speeds associated with the speed limit reduction. Separate logistic regression models estimated changes in the odds of vehicles exceeding 25 mph, 30 mph, and 35 mph associated with the lower speed limit.
Results: The speed limit reduction was associated with a 0.3% reduction in mean speeds (p=0.065), and reductions of 2.9%, 8.5%, and 29.3% in the odds of vehicles exceeding 25 mph, 30 mph, and 35 mph, respectively. All these reductions were statistically significant.
Conclusions: Lowering the speed limit in urban areas is an effective countermeasure to reduce speeds and improve safety for all road users. Practical applications: Local communities should consider lowering speed limits to improve road safety. The current practice of setting speed limits according to the 85th percentile free-flow speeds, without consideration of other characteristics of the roadway, can be a hurdle for local communities looking to lower speed limits. Updated state laws that allow municipalities to set lower speed limits on urban streets without requiring laborious and costly engineering studies can provide flexibility to municipalities to set speed limits that are safe for all road users.