Signalized Intersections: Appendix 6

Considerations for Left-Turn Lane Design

Channelization

A left-turn channelization design should incorporate consideration of the design vehicle, roadway cross section, traffic volumes, vehicles speeds, type and location of traffic control, pedestrians, and bus stops. In addition to these design criteria, consideration should be given to the travel path - drivers should not have to sharply change direction in order to follow the channelization. Channelizing devices should not cause drivers to make turns with angles that vary greatly from 90 degrees. The effect of channelization on pedestrian and bicyclist movement and safety should be closely considered. Additional guidance is provided in the AASHTO Policy on Geometric Design for Highways and Streets.

The type of channelization used for left-turns can improve left-turn movement safety by providing clear separation for left-turning vehicles from the through traffic stream. Also, it guides drivers through an intersection approach, increasing capacity and driver confidence.

Channelization can be provided using curbed concrete or painted islands, or delineators. The appropriateness of raised or flush medians depends on conditions at a given intersection. Painted channelization provides guidance to drivers without presenting an obstruction in the roadway, and would be more appropriate where vehicles may be proceeding through the intersection at high speeds. However, paint is more difficult to see at night, especially at intersections that are not lighted.

Raised curbed islands should provide guidance in the intersection area but should not present a significant obstruction to vehicles. Safety advantages of left-turn lanes with raised channelization include:

• Turning paths are clearly defined within an expansive median opening;

• Improved visibility for left-turning drivers;

• Simultaneous opposing left-turn lanes are offset from one another; and

• Sideswipe collisions due to motorists changing from left to through lanes or vice versa are prevented.

Additional Considerations

Designers should also consider shoulders and access management when designing left-turn lanes:

• Designers should use caution when considering re-striping a shoulder to provide a left-turn lane. Part of the safety benefits of installing the turn lane may be lost due to loss of shoulder, the greater proximity of traffic to roadside objects, and, possibly, due to a reduction in intersection sight distance (ISD).

• Access to properties adjacent to the intersection approach may need to be restricted when a left-turn lane is installed. Signs can be posted to prohibit or restrict left-tuns in and out of the driveways. Also, a median could be installed to implement such restrictions.