Speeding is one of the most common complaints we receive in Engineering Division. Speeding vehicles are annoying, noisy, and can pose a safety threat. On this page we discuss the definition of speeding, how we measure speeds, speed limits, and some tools that we use to reduce speeding in neighborhoods.

How Fast is Too Fast?

The legal definition of speeding is set in the Massachusetts General Law. Any speed faster than the posted speed limit is considered speeding. When no speed limit is posted -- which is most Cambridge streets -- then the maximum allowable speed is the "prima facie" speed. Cambridge is considered "thickly settled" and the prima facie speed is defined as 25 miles per hour. With regard to traffic enforcement, this is the relevant definition.

However, the legal definition of the speed limit -- especially the prima facie limit on neighborhood streets -- feels too fast for most people. In most cases when residents have speeding complaints, we find that most drivers are not exceeding legal limits, but are still moving at a speed that feels too high for the road, as is shown on this map documenting the over 600 speed studies that TP&T has conducted in response to resident complaints. Whether or not drivers are exceeding speed limits, TP&T can help.

How Speeding is Measured

Not every driver travels at the same speed, so describing "how fast" with a single number that adequately illustrates the speed of the street is difficult. When the Engineering Division conducts its own speed study, we measure the speed of 100 vehicles using a radar gun and tabulate the results. The difference between the fastest and slowest vehicle can be as much as 15 MPH. 

Following engineering standards, we use the "85th percentile speed" to describe the speed of the street. This is the speed exceeded by 15% of the drivers. 

We have conducted over 600 speed studies citywide in response to resident complaints.

Posted Speed Limits

The City is not authorized to post a speed limit sign without permission from the state highway department, MassDOT. When petitioned, MassDOT will set the speed limit close to the 85th percentile speed. In other words, we cannot ask MassDOT to set the speed limit we feel most comfortable with. 

Factors Affecting The Speed of Travel

Why do people drive faster on some streets and slower on others? For the most part speeds are a factor of the physical properties of the street and its surroundings. People tend to drive faster on wide, flat, straight, one-way streets with no street parking and with roadside features such as trees and buildings far from the curb. In contrast, speeds are lower on streets that are narrow, hilly, and windy with two-way traffic and roadside objects closer to the curb. Human activity, such as foot traffic and bicycling, noticeably lowers speeds.  Police enforcement is very effective, but only for a short time after the enforcement period ends. Measures that affect speed the least include signs (including speed limit signs), traffic signals, stop signs, and warning beacons.

Ways to Address Speeding

If signs don’t work, what can we do? The most effective way to reduce speeding on a street is to change the street itself. Since the 1990s, traffic engineers have been retrofitting streets with Traffic Calming, which is a set of physical changes to the road. Traffic Calming aims to make the road more friendly to walking and bicycling, which in turn increases the level of human activity. To learn more about the Cambridge Traffic Calming Program, please click here.

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