Traffic calming techniques can be broken down into two major categories:
“permissive” and “restrictive” measures.
Permissive street redesign measures simply slow cars down. In addition to
speed humps, other techniques include:
*Street narrowing: Using a landscaped area to block half of the entrance
to a street alerts the driver to the fact that he or she is on a residential
street where slower speeds are expected. Alternating landscaped areas placed at
midblock minimizes the gun-barrel effect of straight streets, forcing vehicles
to slow down for a one-way passage. Diagonal parking, bicycle lanes, and wider
sidewalks all narrow traffic lanes, slowing cars down.
* Varied pavement surfaces: Different colored paving stones, as on Sixth
Street in Austin, provide visual contrast that captures drivers’ attention and
diminishes the dragstrip appearance of monotonous, straight, and wide streets.
Crosswalks placed over highly elevated speed humps have the same effect, while
giving pedestrians a larger margin of safety.
*Elevated medians: These serve as a refuge for pedestrians crossing
streets, and are also effective in narrowing lanes.
*Traffic circles: Used extensively in Europe in place of signal lights,
traffic circles can reduce speeds without requiring full stops at
intersections. All vehicles approach an intersection in a counter-clockwise
direction around a circle, which may be a landscaped median or merely a circle
painted on the street. Seattle has installed more than 400 traffic circles and
reports reductions in accidents of up to 77% on those streets.
Restrictive street redesign measures, which are common in Europe,
impede motorist access
to a residential area. New suburban subdivisions have been using restrictive
techniques like cul-de-sacs for years:
*Street closure: Blocking the entrance to a street can create an instant
cul-de-sac. An example in Austin is the success of Travis Heights residents in
getting the Riverside Drive entrance to Academy Drive blocked.
*Semi-diverters are used at the ends of streets, allowing only one car to
enter or exit at a time.
*Diagonal diverters convert grid street networks into a series of loops,
shuttling cut-through traffic out of a neighborhood.
Woonerf or“shared street” techniques turn
neighborhood roads into “streetscapes” where vehicles are slowed to a walking
pace. Park benches and islands planted with trees and shrubbery turn the entire
street into an outdoor courtyard. The woonerf concept originated in
Holland in the Seventies when Dutch planners and citizens began to ask why it
was impossible to do anything on residential streets except drive cars.
Woonerf streets have also been designed in Boston, Rochester, New York,
and Portland, Oregon. Portland residents are trying to set up a process by
which city right-of-way can be used as an extension of neighborhood
yards.
“Area-wide traffic restraint” originated in
Germany in the 1980s with the realization that
main roads must be included in any traffic calming plan. Speeds are reduced
to 33 mph on major arterials, 18 mph on neighborhood collectors and 5 mph on
woonerf streets. The primary techniques are speed humps and traffic
circles, but bike lanes, wide sidewalks, medians, and elevated crosswalks are
also used. Results show little decrease in traffic volume, but a decline in
average vehicle speed
from 25 to 12 mph, and an increase in average
journey time of 33 seconds. Traffic fatalities are
cut in half, with a 66% decline in accidents in-
volving children, a 10% decrease in air pollution, and a 14 decible drop in
noise levels. – N.E.
This article appears in June 9 • 1995 and June 9 • 1995 (Cover).