Curb Radius Reduction:

Illustration of actual, and effective, curb radii.

Source: Institute of Transportation Engineers

Photo by Michael Hintze
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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 tend to slow motorists.

Some communities routinely reduce curb radii at locations where the routes: (1) are used by schoolchildren or the elderly, (2) are in neighborhood shopping areas with high bicycle and pedestrian volumes, and (3) are at particular intersections known to have a safety problem (see case study #20). A logical step is to evaluate the curb radii along a corridor frequented by bicyclists, along with a study of the crash types. Care must be used when revising curb radii on routes with truck and bus traffic. If a curb radius is made too small, large trucks and buses may ride over the curb or may veer out into an adjacent traffic lane to make the turn.

When there is parking and/or a bike lane, curb radii can be tighter, because the motor vehicles will have more room to negotiate the turn. Older cities in Europe and in the northeast United States frequently have curb radii of 0.6 to 1.5 m (2 to 5 ft) without suffering any detrimental effects. More typically, however, in new construction the appropriate turning radius is about 4.6 m (15 ft) and about 7.6 m (25 ft) for arterial streets with a substantial number of turning buses and/or trucks. Tighter turning radii are particularly important where streets intersect at a skew. While the corner characterized by an acute angle may require a slightly larger radius to accommodate the turning maneuvers, the corner with an obtuse angle should be kept very tight to prevent high-speed turns.

Purpose

    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.

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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 pedestrians at risk.
    • Curb radii reductions are often used if the functional class of a roadway has changed.
    • Emergency vehicle access should be considered.
    • 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.

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Estimated Cost

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.

Infrastructure
Description
Median
Average
Min. Low
Max. High
Cost Unit
# of Sources (Observations)
Curb/Gutter
Curb
$18
$21
$1.05
$110
Linear Foot
16(68)
Curb/Gutter
Curb and Gutter
$20
$21
$1.05
$120
Linear Foot
16(108)
Curb/Gutter
Gutter
$23
$23
$10
$78
Linear Foot
4(4)

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Case Studies

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