Illustration of actual and effective curb radii. Source: Institute of Transportation Engineers

This modified curb radius reduces the speed of turning vehicles and shortens the pedestrian crossing.
Photo by Michael Hintze

Tighter corner radii reduce crossing distance and slow turning traffic.
Source: Living Streets

Curb Radius Reduction

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 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 feet 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 feet 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.

Purpose

Larger curb radii typically result in high-speed turning movements by motorists, which may increase the risk of pedestrians being struck by right-turning vehicles. Smaller radii can improve pedestrian safety by requiring motorists to reduce vehicle speed by making sharper turns, and shortening pedestrian crossing distances which thereby improves signal timing. Also the smaller radii provide larger pedestrian waiting areas at corners, improve sight distances, and allow for greater flexibility of curb ramp placement.

Considerations

• When designing the actual curb radius based on the effective radius, designs should balance the turning needs of the design vehicle with consideration for nearby land uses and the diversity and prevalence of roadway users. If there are high volumes of large vehicles making turns, an inadequate curb radius could cause vehicles to drive over the curb onto the sidewalk, putting waiting pedestrians at risk.
• Consideration should be given to:
• Adding parking and/or bicycle lanes to increase the effective radius of the corner.
• The angle of the intersection, presence of curb extensions, and the receiving lane width.
• Varying the actual curb radius over the length of the turn to create a compound curve where the radius is smaller, slowing vehicles as they approach a crosswalk and larger after the crosswalk to allow for the turn.
• Curb radii reductions are often used if the functional class of a roadway has changed.
• Emergency vehicle access should be considered.7,8

Estimated Cost

Construction costs for reconstructing tighter turning radii are approximately $15,000 to $40,000 per corner, depending on site conditions (e.g., drainage and utilities may need to be relocated).

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