In Praise of Traffic Calming

By Dom Nozzi

For 50 years, transportation planners have treated streets as little more than conduits for motor vehicles, and see little need for roads other than to maximize motorist driving speeds. Sadly, in all except our remote subdivisions, the quality of life in cities designed for cars has become miserable. No wonder that so many flee the city for the relative safety, peacefulness, and pastoral nature of outlying areas.

According to Cynthia Hoyle, the U.S. has been so successful in providing for fast, unobstructed travel by car that it has seriously undermined the use of transit, walking, and bicycling.

Streets designed primarily with driving speed in mind deter people from walking and bicycling. They’re difficult and unattractive places to walk or bicycle to begin with, and the heavier, faster traffic they generate makes them downright hostile. Pedestrian street crossings are challenging and infrequent, sidewalks are anything but continuous, and anyone who ventures out on a bicycle is soon reminded by an impatient, honking motorist that she’s in the way and doesn’t belong there. “Danger” and “road conditions” or “lack of facilities” are reasons more frequently given in surveys for not bicycling.

How big is the problem for Floridians? One example is the fact that 37 percent of Floridians cannot legally drive—not to mention those who cannot afford to own a car.

The proposed “design speed” for a road affects its dimensions more than anything else. It is the highest speed at which a motorist can drive safely. Not surprisingly, the bible of traffic engineers—”The Green Book”—calls for the design speed, except on local streets, to be as high as practicable.

A wide pavement exerts a strong influence over a motorist. First, it puts someone in a car at a greater distance from objects on either side. Looking at objects that are farther away creates a feeling that a vehicle is moving more slowly and prompts a motorist to compensate by speeding up. Second, by making the motorist survey a broad field in front of his vehicle, a wide pavement provides an assurance that he is in command of that field, which in turn induces him to increase his speed. In addition, when a wide pavement means more lanes, it leaves fewer vehicles in each lane and increases the distance between each vehicle, providing yet another inducement to go faster. Thus an urban arterial with three 11- or 12-foot travel lanes, or a broad two-lane residential street, can have a virtually irresistible effect. Even motorists who are not inclined to drive fast creep up to highway speeds. Others seize the opportunity to floor it.

Cutting down trees, removing other vegetation, taking property by eminent domain, and lowering hills create what traffic engineers assume is the necessary “stopping sight distance.” And the design speed of a road is the primary factor determining the stopping sight distance.

When a traffic engineer states the newly designed road will “improve safety,” beware. While it usually means fewer fender benders, it generally leads to more serious accidents and more accidents involving pedestrians. Making a street “safer” usually tends to increase motor vehicle speeds, which makes the streets less safe for pedestrians or bicyclists. Sixteen percent of all people killed in motor vehicle accidents are pedestrians and bicyclists, which is way out of proportion to the number of pedestrians and bicyclists on the streets. Thirty-nine percent of all children killed in motor vehicle accidents are killed while walking or riding a bicycle. When we hear traffic engineers tell us that the road “improvement” will improve safety, we need to ask them to precisely define what the safety problem is.

Alcohol, vehicle speed, weather, and animals are more important factors in accidents than road design.

Motorists driving at 25 mph or faster have difficulty perceiving that a pedestrian is ready to cross a street, deciding to slow down, and actually doing so. The normal driver usually decides to speed up, assuming that another car will stop.

Many homeowners have essentially written off their front yards as a place to be, largely because of the speed and volume of traffic. It is time that we start designing our communities for people instead of cars. And one of the emerging, exciting ways to do that is through use of “traffic calming.”

Traffic calming involves making design changes to a street or parking lot to slow down and “discipline” autos, and make streets mixed-use “Complete Streets” rather than single (auto)-use. Some strategies include traffic circles (photo above), roundabouts (photo below), on-street parking, narrow travel lanes, reduction in travel lanes, woonerfs, traffic diverters sidewalk bulb-outs, speed humps, smaller turning radii at intersections (15 feet), and elevated/textured/brick crosswalks that serve as a speed hump.

Portland, Oregon has a “skinny streets” program for new residential areas. It allows residential streets to be 20 feet wide with parking on one side, or 26 feet with parking on both sides. The city notes that such streets maintain neighborhood character, reduce construction costs, save vegetation, reduce stormwater runoff, improve traffic safety, and make it possible to use scarce land for purposes other than motor vehicle use. The Portland Fire Department finds that skinny streets provide adequate access for emergency vehicles. It has been noted that it would be more economical to purchase fire trucks that fit local streets than to build all streets to meet the needs of the largest size trucks. Berkeley studies show that traffic control devices had little or no effect on police emergency response time, and Palo Alto found that bicycle boulevard barriers had not impaired police and fire emergency response.

Motorists are more likely to collide with pedestrians at higher speeds. At 60 miles per hour, the field of vision of the motorist is two-thirds less than at 30 miles per hour. In addition, the probability of a pedestrian being killed is only 3.5 percent when a vehicle is traveling at 15 miles per hour, but jumps to 37 percent at 31 miles per hour and 83 percent at 44 miles per hour.

Roadway geometry in safety-sensitive areas, such as schools, should keep auto speeds within 15 to 20 miles per hour. Planting vegetation close to the street will reduce the “optical width” of a street, which makes it seem narrower than it is and help to slow down motorists.

A German study found that traffic calming reduces vehicle idling time by 15 percent, gear changing by 12 percent, brake use by 14 percent, and gasoline use by 12 percent. This is in part because the greater is speed of vehicles in built-up areas, the higher is the incidence of acceleration, deceleration, and braking. Similarly, a study in Portland, Oregon found that a pedestrian-friendly environment can reduce vehicle miles traveled by 10 percent. Other studies show up to a 114-percent increase in non-motorized travel on traffic-calmed streets.

Another German study found that calmed streets experienced a 60 percent reduction in injuries, a 43 to 53 percent reduction in fatalities, and a 10 to 50 percent reduction in air pollution (Nitrogen oxide emissions, for example, begin to increase with speeds at about 15-20 mph, and then increase sharply with speed at about 48 mph.) These substantial benefits, in addition, were achieved by increasing motorist trip time by an average of only 33 seconds. Motorists who found the 18 mile-per-hour speed limit acceptable grew from 27 percent before the streets were calmed to 67 percent after the program began. Receptive residents along the streets grew from 30 percent before to 75 percent after.

Portland finds that traffic circles are most effective when constructed in a series. They are sometimes also located in the middle of the block. Circles reduce motor vehicle speeds and result in a big reduction in the number of accidents. Circles reduce crashes by 50 to 90 percent when compared to two-way and four-way stop signs and traffic signals by reducing the number of conflict points. Seattle likes circles so much that they build about 30 circles each year.

The Institute of Traffic Engineers (ITE) have stated that speed humps are effective in reducing vehicle speeds without increasing accident rates (some studies have found a reduction in accident rate). Humps cause motorists to experience little or no discomfort at speeds up to 25 mph, and need to be spaced close enough to each other so that motorists do not speed between them. The ITE has found that despite concerns about liability, vehicle damage and emergency vehicle impacts, these problems have not occurred or have been found to be insignificant when considering the positive impacts of humps.

And despite the conventional wisdom, stop signs do not affect overall traffic speeds or control speeding. Posting appropriate speed limits and enforcing them is not sufficient to achieve needed reductions in motorist speeds. Modest physical reconfiguration of streets are the only reliable and cost-effective way to slow and control traffic.

Calming also helps reduce neighborhood noise pollution. From a distance of 48 feet, a car traveling at 56 miles per hour makes ten times more noise than a car traveling at 31 miles per hour. Reducing average speed from 25 miles per hour to 12 miles per hour reduces noise levels by 14 decibels (ten times quieter). At higher speeds, every 12 to 15 miles per hour in speed increases results in a 4 to 5 decibel noise increase.

The City of Oakland recently budgeted $1 million to install traffic calming measures throughout the city in response to citizen petitions for safer streets. The City has already installed speed humps and is pursuing road narrowing and barriers to through traffic. A similar strategy in Menlo Park has reduced through traffic by 66 percent, has reduced top speeds by 40 percent, and has reduced average speed by 20 percent.

It is important to learn from our past in designing street intersections. For example, in the past, we designed corners with a small “radius.” A corner with a radius of 15 feet or less is usually appropriate to require turning vehicles to slow down, and also shortens the distance that a pedestrian must walk to get across the street.

A maximum driving speed of 19-25 mph is necessary to ensure safety, create an environment people find conducive to walking and shopping, and minimize noise. Fred Kent, a nationally known urban designer, says that in all the surveys he has done around shopping districts, the biggest problems are not security issues. They are traffic issues—the speed of vehicles, the noise of vehicles, the congestion. You realize that if you create less vehicle flow and slower vehicles, you create more of a sense of community and you increase the perception of safety and security.

Here are some of the benefits that a German city found by using traffic calming:

· 50 percent increase in bicycle use.

· 57 percent reduction in fatal accidents.

· 45 percent reduction in severe accidents.

· 40 percent reduction in slight injuries.

· 43 percent reduction in pedestrian accidents.

· 16 percent reduction in cyclist accidents.

· 16 percent reduction in traffic accident costs.

· 66 percent reduction in child accidents.

The Federal Highway Administration (FHA) has stated that traffic calming appears to be one of the more cost-effective ways to promote pedestrian and bicycle use in urban and suburban areas, where walking and bicycling are often hazardous and uncomfortable. By improving the quality of urban neighborhoods, traffic controls can help reverse the flight of the middle class away from the city. And as for children, Stina Sandels, a world authority on children and road accidents says that the best road safety education cannot adapt a child to modern traffic, so traffic must be adapted to the child.

The FHA notes that the importance of reducing traffic speed cannot be overemphasized. While the overall goals of slowing traffic may include environmental improvements, better conditions for bicyclists and pedestrians, accident reductions, and more space for children to play-the reduction in vehicle speeds is crucial to each.

The primary question has become whether or not the city, which was formerly built on the human scale, and in which the street existed primarily as a means of contact, is to be replaced by a sprawled megalopolis where the dimensions of the street and city are on a scale required for its primary use by motorized transportation, and whether we will let our quality of life and sustainability remain terrible—all in the name of making cars happy.

References:

Traffic Calming by Cynthia Hoyle

Traffic Calming by CART (David Engwicht)

Sustainable Community Transportation by Todd Litman

Taming the Automobile by Richard Untermann

Take Back Your Streets by the Conservation Law Foundation

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Visit my urban design website read more about what I have to say on those topics. You can also schedule me to give a speech in your community about transportation and congestion, land use development and sprawl, and improving quality of life.

Visit: www.walkablestreets.wordpress.com

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