Peter Lagerwey, Pedestrian & Bicycle Program Coordinator, City of Seattle
When streets intersect at an obtuse angle or have a large curb radius, motorists can make turns at relatively high speeds. By contrast, 90-degree intersections and corners with tight curb radii tend to slow motorists down. The problem with obtuse angles is particularly bad when a vehicle on an arterial street turns onto a residential street. Motorists turning right at high speed may cut off bicyclists traveling straight on the arterial street. Pedestrians crossing the residential street adjacent to the arterial may not expect high-speed turning traffic, or they may have their backs facing the turning cars.
The solution to this problem in Seattle has been to reduce the turning radius. Seattle routinely reduces the curb radii at locations that: a) are on routes used by school children or the elderly; b) are in neighborhood shopping areas with high pedestrian volumes; and c) are at intersections identified by the neighborhood as having a unique safety problem.
Obtuse angle intersection allowed motorists to make high-speed turns.
The goal is to slow down right turning motor vehicles. This solution works particularly well where motor vehicles are turning right, at an obtuse angle, from an arterial street onto a residential street.
When making curb radii revisions, consideration must be made for truck and bus traffic. A curb radius that is too tight may result in the truck or bus crossing the double yellow line or overriding the curb. This can damage the curb and pose a risk to pedestrians. However, when a truck or bus is turning onto a four-lane roadway (two lanes in each direction), it often is acceptable to turn into the second (inside) lane as long as the center double yellow line is not crossed. Such turns would not be acceptable in cases where truck traffic is very heavy or there is a double right turn.
Seattle has adopted the following guidelines for reducing curb radii:
Reducing the curb radius is expected to reduce turning speeds and increase the comfort of bicyclists traveling straight through past this junction. Seattle has not conducted a formal study to determine if crash rates have been reduced.
Curb realignment reduced the turning radius, forcing turning vehicles to slow. Crossing distance was also narrowed.
While many transportation agencies have increased curb radii over the years, these changes have had the effect of increasing the turning speed of motor vehicles. This has made bicycling and walking less safe and less inviting. In many cases, turning radii have been unnecessarily increased on neighborhood and arterial streets where there is little or no truck or bus traffic. Seattle has found that reducing curb radii is a relatively cheap, effective and popular way to create a more bicycle- and pedestrian-friendly community.
The costs of changing curb radii can vary considerably, depending on the amount of concrete and landscaping that is required and also on whether drainage grates and other utilities have to be moved or if there are other issues that need to be addressed. For example, it may be necessary to move a conduit for a signal or relocate utility poles and light standards. In Seattle, costs typically range from as low as $5,000 to as high as $40,000.
Bicycle & Pedestrian Program Coordinator
Seattle Department of Transportation
700 Fifth Avenue, Suite 3768
P.O. Box 34996
Seattle, WA 98124-4996