One city’s traffic speed data

One job of a municipal transportation department is answering “speeding” complaints. Some cities respond by collecting speed data to see if the complaint is really about speed or about unwanted traffic.

Often the response doesn’t see the light of day, but Cambridge, Massachusetts has released 28 years of data collected in response to such complaints. The speed studies page has an interactive map of 600 speed studies.

The map tells you 85th percentile speed, which is a few miles per hour over the average speed. The speed limit should be close to the 85th percentile speed.

If you see a number around 25 or 35, that tells you the standard 30 mph speed limit is wrong for a street.

The high numbers are on streets where I already knew there was a speed limit problem. If you don’t know the area you’ll have to check a map program or street view to see why. Binney Street is four lanes divided, Concord Avenue west of the rotaries is three lanes with few curb cuts, the west end of Massachusetts Avenue is four lanes divided. These should have limits over 30.

The flags on the map correspond to resident complaints, and cluster along residential streets. Many of those have slow traffic and could legitimately be posted 25.

Cities keep asking the state to lower the speed limit based on all those residential areas, but if the speed limit drops to 25 police are going to be heading to those main roads to make quota.

A blanket speed limit within city limits doesn’t work.

Some states exempt arterial streets from standard speed limits. For example, California’s “speed trap” law effectively requires speed limits on major city streets to be justified by an engineering study, not simply by reference to the statutory speed limit.

If we had a California-style “speed trap” law in Massachusetts we might be able to give cities a 25 mph limit in truly urban areas without creating speed traps.

But most of the people asking for speed limit reductions want to create speed traps.

The opinions expressed in this post belong to the author and do not necessarily represent those of the National Motorists Association or the NMA Foundation. This content is for informational purposes and is not intended as legal advice. No representations are made regarding the accuracy of this post or the included links.

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