3.7C Curb-return radii

Use smallest practical radius

The smallest practical curb-return radius should be used at all intersections to manage the speed of turning vehicles, shorten the length of pedestrian crosswalks, and provide adequate space at the corner for properly aligned curb ramps. It is important to choose a practical radius as selecting design and control vehicles smaller than appropriate can compromise the safety of people biking and walking, as large vehicles may be required to frequently mount the curb to complete a turn.
 
Choose the smallest practical radius with all other intersection safety improvements included. Sometimes implementing intersection safety treatments may result in wider curb radii, but an overall safer street (e.g. a median or curb extension often requires wider curb radii). Designers should weigh the balance of wider radii with other safety treatments in each situation.

Extra attention should be paid to tightening curb-return radii where:

• There are adjacent or nearby trip generators where high pedestrian volumes are observed or anticipated;

• Turning vehicles will cross a protected bike lane, shared use path, or trail;

• There is no existing or anticipated future need to regularly accommodate large turning transit buses or trucks; and 

• Where adjacent parking lanes, on-street bike lanes, or striped shoulders create a larger effective radius.

Use design and control vehicle guidance

The design and control vehicles identified for an intersection inform the curb radii that should be used.

Where turns are prohibited

Where turning movements are prohibited, such as at one‐way streets, a 5’ corner radius should be used.

Alleys

A 5’ corner radius is typically used for alleys.

Actual vs. effective radius

The actual curb-return radius may differ from the effective radius when on-street parking, an on-street bike facility, or a striped shoulder is adjacent to the outside travel lane (see Figure 3.7C.2). AutoTurn movements should consider the effective radius and a tighter actual radius should be implemented whenever practical.
Figure 3.7C.2: Actual vs. effective curb radius

Strategies to tighten radii

To achieve the smallest practical curb-return radii, the following design strategies may be considered:

• Implementing a compound radius so that the radius is smaller as drivers approach the crosswalk and larger as they continue through the intersection to complete the turn;

• Implementing an advanced stop bar so control vehicles can encroach into the opposing or adjacent travel lane to complete a turn; and

• Implement an access or turning restriction depending on adjacent land use and traffic network needs.