copyright 1996, Tracy-Williams Consulting
If a residential street is being used by uncomfortably high volumes of potentially fast traffic, traffic calming measures may be called for.

What is traffic calming?

There are many traffic calming approaches in use around the world (generally, the U.S. lags behind places like the Netherlands and Australia in this field). Traffic calming, simply put, involves physical measures intended to (1) slow motor vehicle traffic down; (2) discourage motorists from using a particular street or network of streets; (3) or both.

Slowing traffic is typically accomplished through the use of raised sections of roadway, as in a speed table, or stretches of road that are narrower than normal, as in a squeeze point or so-called "skinny streets", or stretches that shift the road's alignment, as in a chicane. Another popular measure for slowing traffic is the residential street traffic circle. Different agencies prefer different measures. For instance, the City of Portland has been moving towards speed humps while Seattle often uses traffic circles.

The photos below show several typical traffic calming measures used for speed control.

photos courtesy City of Portland

While such speed-control measures may discourage some through traffic, other approaches are typically used for this purpose. For instance, intersection diverters keep motorists from going straight and force them to turn--typically to the right. Partial diverters only affect one direction of travel. These may allow traffic to leave a particular street via a specific intersection but not allow it to enter. Midblock barriers are also used to stop through traffic. The photo below shows just such a midblock diverter.


photo courtesy City of Seattle

Structures like that shown above can help in the creation of bicycle boulevards,which are low-volume alternate routes adjacent to major arterials. The City of Palo Alto, California, for instance, has installed such bicycle boulevards and many bicyclists appreciate the option of using quiet streets to get where they need to go. In their report on the subject, the City noted a substantial increase in bicycle traffic on their bicycle boulevards after installation of traffic diverters similar to the one shown above.

NOTE: It's important to keep in mind that traffic calming measures can cause bicycling hazards, if they are misapplied or badly designed. Reports from the New Jersey Department of Transportation, the U.S. Department of Transportation, and England's Cyclist Touring Club, discuss ways that bicycle-friendly traffic calming measures can be used.

What isn't traffic calming?

To many neighborhood residents, slowing traffic down means using either stop signs or old-style speed bumps. However, public works departments don't like to use these.

In general, stop signs should not be used as speed control devices. The real reason to install a stop sign is to assign right-of-way between competing traffic. For example, stop signs are used to stop residential street traffic when it reaches a collector street. Studies suggest that using stop signs for speed control doesn't really work very well anyway since motorists typically accelerate quickly after stopping--if they stop at all.

Speed bumps--like those found in mall parking lots and mobil home parks--are also popular, if inadequate, measures. The typical speed bump spans the roadway and is roughly 1 foot in length. This contrasts with the "speed table" which is much longer. Speed bump height varies but 6 inches is typical. Here's the trick with speed bumps: in many cars, the faster you go, the less you feel them. This, unfortunately isn't true for other road users.

Back in the 1970s, engineers from the City of San Jose, California, conducted a series of tests with standard speed bump designs. Their results were written up in Traffic Engineering magazine and they showed that bicyclists, motorcyclists, and emergency vehicles typically had trouble with speed bumps. The bicyclist hit a pedal on the bump. The motorcyclist nearly lost control. And the fire truck went airborn.

How do you use traffic calming?

Traffic calming must be seen as a neighborhood-wide and, preferably, a community-wide program. Experience in cities like Eugene and Portland, Oregon, and Seattle, Washington, suggest that traffic calming one street or installing one traffic circle may simply shift the problem to an adjacent street or intersection.

The best way to use traffic calming techniques is by creating an on-going program that, in cooperation with local neighborhood groups, identifies residential street traffic problems and solves them in a prioritized manner. One such program is that found in Seattle. And they offer an informative video, as well as print information, about their process. Among other things, the video discusses the role of neighborhood groups in implementing traffic circles on residential streets.

What are the benefits?

Traffic calming can provide residents with quieter and less hazardous neighborhoods. Studies in Seattle, for instance, have shown dramatic reductions in motor vehicle crashes after traffic calming.

Another perhaps less obvious benefit is a shift in the priorities for residential street use. Since neighborhoods are, first and foremost, places where people live, it's important to keep automobile traffic in check. Children should not, for instance, be threatened by fast moving traffic just a few yards from where they play. And motorists should be discouraged from using residential streets as bypasses for busy arterials.

References:

Topics for further study:

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