Bicycle Safety Guide and Countermeasure Selection System
Illustration of actual, and effective, curb radii. Source: Institute of Transportation Engineers
Motor vehicles turning at a high rate of speed pose problems for bicyclists (as well as pedestrians). This is a common problem when motorists traveling on an arterial street turn onto a residential street. A typical bicycle-motor vehicle crash type, sometimes called a "right hook," occurs when a motor vehicle passes a bicycle going straight ahead and then turns right shortly after making the passing maneuver. Reducing the radii of curbs at these high speed right turns provides a remedy. Creating 90-degree intersection corners, or corners with tight curb radii, tends to slow motorists.
Curb radii designs are determined based on the design vehicle of the roadway (i.e. the types of vehicles using the roadway, such as buses, tractor trailer trucks, fire trucks, etc.). The most important factor for design is using the effective radius rather than the actual radius to accommodate the chosen design vehicle. Actual curb radius refers to the curvature along the curb line; effective radius refers to the curvature vehicles follow when turning. Larger effective curb radii can be achieved by adding on-street parking, bicycle lanes, or by striping advance stop lines on the destination street of multilane roadways.
The smallest practical actual curb radii should be chosen based on how the effective curb radius accommodates the design vehicle. An actual curb radius of 5 to 10 ft should be used wherever possible. An appropriate effective radius for urban streets with high volumes of pedestrians is 15 to 20 ft. For arterial streets with a substantial volume of turning buses and/or trucks, an appropriate effective curb radius is about 25 to 30 ft. Typically the maximum desired effective curb radius is 35 ft for large vehicles. Tighter turning radii are particularly important where streets intersect at a skew. Corners characterized by an acute angle may require a slightly larger radius to accommodate larger vehicles; corners with an obtuse angle should have the smallest feasible radius to prevent high-speed turns.
Motorists awareness of bicyclists during right turns can be improved by creating a safer intersection design. Larger curb radii typically result in high-speed turning movements by motorists, which may increase the risk of bicyclists being struck by right-turning vehicles. Smaller radii can improve safety by requiring motorists to reduce vehicle speed by making sharper turns.
Costs for reconstructing a curb to a tighter radius can vary from approximately $5,000 to $40,000, depending on site conditions (e.g., the amount of concrete and landscaping that is required; whether drain grates and other utilities have to be moved; and whether there are other issues that need to be addressed).
Curb and gutters are used in conjunction with a number of other bicycle and pedestrian facility improvements, such as: sidewalks, bikeways, medians, islands, paths, curb extensions, bikeways, diverters, chicanes, and bulb-outs, among others. The cost can vary widely based on the scale of the project and whether the curb and/or gutter installation is in conjunction with other road treatments.
