Traffic Calming FAQ

What is traffic calming?

"Traffic calming is the combination of mainly physical measures that reduce the negative effects of motor vehicle use, alter driver behavior and improve conditions for non-motorized street users." Unlike traffic control measures such as speed limit signs and stop signs, traffic calming measures are intended to be self-enforcing. "Citizens are often concerned about excessive traffic volumes and speeds on residential streets. Local streets are intended to serve the adjacent land use at slow speeds, yet they are often designed so that high speed travel is accommodated. Well-designed traffic calming devices effectively reduce traffic speeds and volumes while maintaining local access to neighborhoods."

You can also read more about calming here!

Isn't more police enforcement what we really need?

Unfortunately, enforcement only works while the officer is present. Within a few minutes of his/her departure, speeding, rolling stops, and other unsafe driving practices return. The real problem is that our roads are engineered for much higher speeds than we want traffic to move. Without physical changes, enforcement will be a continuing, expensive battle.

Radar trailers are a bit less expensive but also effective only when present. Photo-radar enforcement systems are slightly more effective, but they are expensive to operate since they need staffing.

Why don't we just add more stop signs?

Stop signs were one of the original "traffic calming" attempts. Palo Alto seems to have more stop signs per mile than anywhere else. So we should all know the answer to this one: stop signs don't work as a calming device!

Apart from being annoying to (and usually ignored by) cyclists, nearly 60% of all drivers fail to come to a complete stop at a stop sign. At four-way stops, they also tend to assume that they have the right to go as soon as they have slowed/stopped, increasing risks for pedestrians and cyclists.

They also provide little or no mid-block speed reduction (and in some cases, increase speed mid-block!) and increase noise levels near the intersection.

What kinds of things are in the Traffic Calming Toolkit?

There are two major classes of traffic calming devices:

Volume Control - this group includes various full and partial closures and is intended primarily to reduce the volume of cars on a street. Common types of Volume Controls are: full closures, half closures, semi-diverters, diagonal diverters, and median barriers.
Speed Control - this group includes three types of control measures and is intended primarily to reduce the speed of cars on the street. As a side effect, installation of speed control measures usually reduces cut-through volume as well. The three catagories of speed control are:
- vertical measures - speed humps, speed tables, raised crosswalks, and intersections and textured pavements.
- horizontal measures - traffic circles, chicanes, lateral shifts.
- narrowings - neckdowns, chokers, bicycle lanes, center islands
Modern roundabouts are similar to traffic circles but with added safety features.

Narrowings are called psycho-perception controls which work by making the driver feel that they have less space. They are less effective than other speed control measures which rely on physical forces.

How well do they work?

Some devices work better than others and all require proper engineering to have the desired effect!

Volume control devices are typically very effective... on the streets where they're located. As we know from experience, however, any "leaks" will be found and used.

Speed humps produce a 10 - 20% reduction in speed when properly designed and spaced. Circles are also very effective, reducing overall speed by about 11%. Speed control devices such as humps and circles also tend to reduce traffic volume by up to 15% but this is dependant upon the perceived time delays on the alternate routes. As congestion around our neigbhorhood continues to increase, the effect of calming devices on cut-through traffic will continue to errode.

Don't these devices increase the noise? Pollution?

It is true that speeding drivers may make noise braking sharply when they "discover" the first hump! And there is a small increase in noise "downstream" from vertical devices, particularly when cargo-carrying trucks make up a significant fraction of the traffic stream. However, in general experience indicates that the lower speeds tend to lower noise levels! Horizontal measures generally create less noise than vertical measures and for that reason might be preferred on streets with large numbers of trucks.

Comprehensive traffic calming of German towns in the 1980's resulted in decreaced air pollution. This is consistant with the general understanding that "slow and steady" traffic produces less pollution than "speed, stop and sit". More recent studies in the U.S. support this conclusion.

Are these things safe? What about emergency response?

Safety is probably the biggest benefit of traffic calming! Although the U.S. figures are not as impressive as the European experience, in all cases the numbers of accidents declined by over 50%!

Traffic circles have a particularly impressive record, reducing collisions by up to 82%. And even when collisions occur, they are far less severe because they tend to be at lower speed and glancing (instead of head on or T-bone) impacts. Pedestrian safety is also improved because there are less points of possible impact at a traffic circle than at a conventional intersection.
Potential Conflicts Reduced by Traffic Circles.
Source: H. Stein et al., "Portland's Successful Experience with Traffic Circles," in 1992 Compendium of Technical Papers, Institute of Transportation Engineers, Washington, DC, 1992, pp. 39-44.

Chicanes and humps also appear to reduce collisions by a large margin.

Vertical calming devices receive more resistance than horizontal devices from emergency response teams because of the jarring effect they have on equipment and patients. However, the measured delay per calming device is typically less than 10 seconds with horizontal devices having slightly more delay. Care is needed not to install too many devices on primary response routes and some design alterations to devices are possible to decrease the impact on emergency response equipment.

How much do they cost?

Among the speed control devices, simple humps are the least expensive (~$2,500) and traffic circles the most expensive ($10,000 - $15,000). Nice landscaping adds to the cost of circles. Closures, especially if landscaped, are also pricey - up to $100,000. A single "movable bollard" of the type used at Stanford is $30,000! So there is no "standard cost" for calming devices.

Aren't they ugly?

Unfortunately, the aesthetics of traffic calming devices are largely a trade-off for cost! Raised devices can range from simple asphalt to much more elegant patterned paving. With enough money, circles, closures, and restrictors can be beautiful landscaped features. Some citys use neighborhood volunteers to maintain the landscaping though there are liability concerns. This cuts the cost but may be more or less effective.

What is a Woonerven?

European traffic calming began as a grassroots movement in the late 1960's when angry residents of the Dutch city of Delft began turning their streets into "living yards" or "woonerven" to combat cut-through traffic. They placed tables, benches, sand boxes and parking bays in the street, turning it into an obstacle course for motor vehicles and an extension of home for residents.

Woonerf design is not a cure-all: it is expensive and is unsuitable for streets with large amounts of traffic. Cars move through at about 15 mph which is sustainable only for short distances on local access streets. On the other hand, it has a wonderful effect on livability of the street and may actually be applicable to some of our College Terrace streets. Many of our "full closure" streets already tend to function this way!

Traffic Calming: State of the Practice, Federal Highway Administration, 1999

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