Bicycle Safety Guide and Countermeasure Selection System

 

Curb Radii/Curb Revisions

Seattle, Washington

Prepared by Emily Ehlers, Senior Transportation Planner, Seattle Department of Transportation

Background

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. High-speed right turns from arterial streets onto residential streets can be especially unsafe if motorists cut off bicyclists traveling straight on the arterial street. Likewise, 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.

Obtuse angle intersection allowed motorists to make high-speed turns.

Obtuse angle intersection allowed motorists to make high-speed turns.

Countermeasures

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.

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. Other operational factors include presence of on-street parking, number of lanes, and one-way or two-way traffic. In addition, adjacent land use and whether there is a marked pedestrian crossing or school crossing at the intersection affect the curb radius.

Seattle's Right of Way Improvement Manual includes the following guidelines for reducing curb radii:

Evaluation and Results

Seattle has not conducted a formal study of crash rates, but reducing the curb radius is expected to reduce turning speeds and increase the comfort of bicyclists traveling straight through the junction.

Curb realignment reduced the turning radius, forcing turning vehicles to slow. Crossing distance was also narrowed.

Curb realignment reduced the turning radius, forcing turning vehicles to slow. Crossing distance was also narrowed.

Conclusions and Recommendations

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.

Costs and Funding

The cost of changing curb radii can vary considerably, depending on the amount of concrete and landscaping required, 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.

Contact

Emily Ehlers
Senior Transportation Planner, Bicycle and Pedestrian Program
Seattle Department of Transportation
700 5th Avenue, Suite 3700
P.O. Box 34996
Seattle, WA 98124-4996
(206) 684-8264