Curb
Radius Reduction
Description
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 five to 10 feet should be used
wherever possible. An appropriate effective radius for urban streets with high
volumes of pedestrians is 15 to 20 feet. For arterial streets with a
substantial volume of turning buses and/or trucks, an appropriate effective
curb radius is about 25 to 30 feet. 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
becoming 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.
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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