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Corner radii directly impact vehicle turning speeds and pedestrian crossing distances. Minimizing the size of a corner radius is critical to creating compact intersections with safe turning speeds. While standard curb radii are 10–15 feet, many cities use corner radii as small as 2 feet.
In urban settings, smaller corner radii are preferred and actual corner radii exceeding 15 feet should be the exception.
A large corner radius should not be used to facilitate a truck turning from the right lane into the right lane.2
The distinction between the corner radius and the effective turning radius is crucial and often overlooked. The corner radius may be a simple or a complex curve and depends primarily on the presence of on-street parking, bike lanes, the number of travel lanes, medians, and traffic control devices.
Designers often determine corner radii based on the intersection geometry only and overlook the effective radius. As a result, drivers making a turn on a green signal have little incentive to turn into the nearest receiving lane and routinely turn as wide as possible to maintain travel speeds.
See Interim Design Strategies
Kendra K. Levine, Curb Radius and Injury Severity at Intersections (Berkeley: Institute of Transportation Studies Library, 2012), 2.
Joseph E. Hummer, Charles V. Zegeer, and Fred R. Hanscom, Effects of turns by larger trucks at urban intersections, (Charlotte, N.C.: Transportation Academy, Dept. of Geography and Earth Sciences, University of North Carolina at Charlotte, 1988).
Robert Kahn and Allison Kahn Goedecke, “Roadway striping as a traffic calming option,” ITE Journal: 81 (September 2011).
Adapted from the Urban Street Design Guide, published by Island Press.