Road Management & Engineering Journal
Road Management & Engineering Journal
May, 2001
TranSafety, Inc.
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Caution, Speed Bumps Ahead
by Phyllis Orrick

(This article is reproduced, with permission, from the summer 1999 issue (No. 66) of Tech Transfer: Technology Transfer for Local Transportation Agencies, a publication of the University of California's Institute of Transportation Studies Technology Transfer Program.)

The demand for traffic calming is rising, but making streets safer and more comfortable for people is still largely unmapped terrain.

Traffic calming has had a bumpy history since it was introduced from Europe into a handful of United States cities some 30 years ago, and the controversy continues. For instance, in the city of Berkeley, which was among those to pioneer traffic calming, leaders in the disabled community say that they can't travel over a speed hump in a vehicle without excessive pain; therefore, speed humps might violate the Americans with Disabilities Act.

Assistant fire chief Mike Migliore, along with civilian supporters of emergency service providers, has objected to speed humps and traffic diversions on the grounds that they slow emergency response time and endanger residents. The controversy has choked off the growth of Berkeley's extensive traffic-calming initiatives, to the point where the city imposed a near-total moratorium in 1995.

But while Berkeley's efforts have been frozen, many other cities are looking at ways to implement traffic calming measures when growing numbers of vehicles take to local streets to avoid clogged arterials. Even more jurisdictions are likely to consider traffic calming now that federal funding is available for it. The 1998 Transportation Equity Act for the 21st Century (TEA-21) specifically names "traffic calming" as a program eligible for funding. It's the first time this category has been identified by Congress in this type of bill.

Accompanying this trend, traffic calming has been the subject of numerous studies in recent years. One of the most recent and comprehensive is Effective Traffic Calming Applications and Implementation, published by the Minnesota Local Road Research Board of Minnesota's DOT. This report defines "traffic calming" as the deployment of physical devices that alter the roadway with the objective of changing drivers' behavior. While traffic control devices such as stop signs or signals certainly control driver behavior, traffic calming works by creating an environment where drivers will be convinced to slow down or go elsewhere. Diverters literally steer drivers to alternate routes while other more subtle environmental effects create a sense of shared space, causing drivers to move more slowly and cautiously on the "calmed" route.

The Minnesota study criticizes agencies for past traffic-calming efforts because they failed to provide the data necessary to properly evaluate them. It recommends collecting pre- and post-implementation data about driver behavior to better analyze the effectiveness of traffic-calming measures and to help other agencies that might adopt them. Parameters that should be considered include average daily traffic counts, vehicle speeds, accident rates, residents' reactions, roadway classification, funding sources and effects on service providers. Data limitations notwithstanding, this study undertakes an evaluation of the advantages, drawbacks and effectiveness of 21 traffic- calming practices. The most promising commonly considered follow:

Speed Humps

The speed hump is the most commonly used traffic-calming device in the United States according to last year's A Survey of Traffic Calming Practices in the United States, by Asha Weinstein and Elizabeth Deakin. The officials whom Weinstein and Deakin interviewed for their survey, representing 45 localities, said they chose speed humps because they are affordable ($1,000 - $3,500 apiece), and they are dramatically effective at slowing traffic.

Speed humps work best in pairs no more than 250 feet apart on streets two lanes wide or less. They are mounds, usually made of asphalt, that run from curb to curb and have a flat top that is three to four inches high and 6 to 8 feet across. No more than two pairs should be installed every half-mile. Their drawbacks are that they slow fire trucks and other large vehicles like garbage trucks and buses and are difficult to cross safely for bicyclists and some disabled people riding in cars or vans. Speed humps should not be installed on busy streets because they are too disruptive of traffic flow or on transit routes because of their effect on buses. Many cities have banned them from emergency response routes. Other shortcomings include possible increases in traffic noise from braking and acceleration.

Speed Tables

Speed tables, unlike humps, don't stretch from curb to curb, making it easier for fire trucks and bicyclists to pass without going up and over them. Their top is wider, typically 12 feet or more, enabling both the front and back wheels of a standard vehicle to be on the table at the same time. They can be traversed comfortably at 15-25 mph, a little faster than speed humps, and they too can cause increases in traffic noise from braking and acceleration.

Traffic Circles

Traffic circles force drivers to slow down by exerting lateral motion on vehicles as they go around the circle. The circle must be at least 12 feet across to be effective. Usually, they are formed around a raised, landscaped island. Circles are most effective at reducing left-turn and sideways collisions. Singly, they only have a short-term effect on traffic speed, since drivers tend to reaccelerate quickly after passing through. However, if a series of them is constructed, they can noticeably slow traffic. They can be costly, $7,000 and up, depending on landscaping and curb design. Their disadvantages include: causing conflicts between bicycles and motorized vehicles by forcing them to share narrower traffic lanes; reducing access by emergency vehicles if vehicles are illegally parked on or approaching the circle; and a loss of parking since they require roughly 30 feet of clear curb on both sides of each entry.


There are no United States DOT guidelines for roundabouts, but they are generally no smaller than 50 and no larger than 115 feet in diameter. They work best at increasing flow and capacity at an intersection by imposing a yield-at-entry rule, which means vehicles in the circle have the right-of-way over those entering, which are controlled by a YIELD sign, before being slowed and deflected to the right by the central island. Roundabouts can calm traffic indirectly by changing the character of a roadway as it goes from a busier, commercial district to a residential one. Their landscaping can also help to calm traffic. However, they can be extremely expensive to build, starting at $10,000. Their spatial demands can require removal of parking spaces and create difficulties for pedestrians.

Chicanes and Chokers

Chicanes also change a roadway's character. They are essentially islands or curb extensions built into the street in alternating pairs, like the teeth of a zipper, which force drivers to slow their cars. They provide opportunities for landscaping and refuges for pedestrians, both of which calm traffic by reminding drivers to share the roadway. They work best on low-volume streets and must be designed with care to prevent bicycles from being forced into too close proximity to motorized vehicles. Chokers are scaled-down chicanes and work singly.

Route Modifications

These are cul-de-sacs, diagonal diverters, semi-diverters and medians. They require extensive pre-implementation study of their possible effects to avoid spillover on nearby streets. They are also generally very costly and their use must be reconciled with the objections of emergency and other services providers who will lose direct access to the closed off blocks.

Mechanized Enforcement

Mechanized traffic rule enforcement is gaining popularity. The best-known in use today is the Motion Imaging Recording System (MIRS), deployed in 13 states (including California). Its most common use is to catch red-light runners. A camera snaps pictures of an offending vehicle's license plate as it passes through the intersection, and the registered owner subsequently receives a summons or warning in the mail. The system's effectiveness has yet to be documented; its biggest drawback is political unpopularity among those who consider it an invasion of privacy.

On-Street Angled Parking and Protected Parking Bays

These narrow the roadway, causing drivers to slow down. Bays can be built around parallel end-to-end spots, or parking can be switched to angled, on-street spaces. However, such arrangements can increase danger for pedestrians, especially children, as they walk between parked cars, and, if there is only enough room to put the new configuration on one side of the street, parking capacity is cut, which could lead to illegal parking and added danger.

When Traffic Calming Devices Don't Make Sense

Another recent entry in the traffic-calming literature is the Traffic Calming Primer, published earlier this year [1999] by Pat Noyes & Associates. Pat Noyes, the lead author, says that in some cases the best traffic-calming devices are no traffic- calming devices. That happened in the city of Pleasanton, California, which hired Noyes to consult on a citywide traffic-calming plan. In one neighborhood, residents could not convince those a few blocks away (who had to pass through the trouble spot to get to their homes) to support traffic-calming devices. Instead, they created an educational campaign for their own problem blocks, using eye-catching posters on their curbside garbage cans asking motorists to slow down and share the street, as well as a temporary program of stepped-up enforcement. Residents with the traffic problem were happy with the resulting neighborhood solidarity and calmer traffic, and those less immediately affected did not have their driving patterns and streets disrupted.

The final word then is one of caution. Consider carefully the parties who will be affected by implementing traffic-calming measures, and be as inclusive as possible. Ultimately, the best effort might be limited to education and outreach.

Recommended Reading

Effective Traffic Calming Applications and Implementation by Amy J. Saffel (Braun Intertec Corporation), Minnesota Local Road Research Board, Minnesota DOT, available from: National Technical Information Services, Springfield, VA 22161.

A Survey of Traffic Calming Practices in the United States by Asha Weinstein and Elizabeth Deakin, WP 703, Institute of Urban and Regional Development, March 1998, UC Berkeley.

Traffic Calming Primer by Pat Noyes & Associates, 1566 County Road 83, Boulder CO 80302.

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