Corner Radii

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 corner radii are 3–5 m, in urban settings, smaller corner radii of 1.5 m are preferred and corner radii exceeding 5 m should be the exception.

Overview

The size of the corner relates directly to the length of the pedestrian crossing. Longer pedestrian crossings take more time to cross, increasing pedestrian exposure to risk and diminishing safety.

A smaller corner radius expands the pedestrian area, allowing for better pedestrian ramp alignment.

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. Corner radii are often 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.

Design Guidance

Turning speeds should be limited to 10 km/h or less. Minimizing turning speeds is crucial to pedestrian safety, as corners are where drivers should expect to encounter pedestrians crossing.

Various methods that accommodate large vehicles, while restricting the turning speed of smaller vehicles, may be used to avoid unnecessary widening of the intersection. Minimize effective turning radius where possible by employing one or more of the following techniques:

• Select the smallest possible design vehicle.
• Accommodate trucks and buses on designated truck and bus routes.
• Restrict right turns on red so there is no expectation of turning into the nearest receiving lane.
• Require larger vehicles to employ roadway personnel to “spot” vehicles through difficult turns.
• Design so that emergency vehicles may utilize the full area of the intersection for making turns.

In cases where the curb radius of a given intersection has resulted in an unwieldy or unsafe crossing distance, but where funding is not available to reconstruct the curb immediately, a city may delineate the appropriate curb radius using interim materials such as epoxied gravel, planters, and bollards. This should be a temporary option until funding becomes available for a more permanent treatment.

Narrow streets with curbside travel lanes may require larger corner radii because the effective turning radius mirrors the actual corner radius. The same holds true for streets with curb extensions. Streets should not be designed with larger corner radii in anticipation of the entire roadway being used for vehicle traffic at some point in the future.