NACTO Urban Street Design Guide Write up on Left Turn Phasing

ADDED: FEBRUARY 10, 2014: Atlantic Cities did a post titled "Which Traffic Signals are Best?" The same language problems I tried to correct with Signal Phasing in these edits were from the original research on the topic.

I edited the Split Phasing page with some general guidance on "Left Turn Phasing Options". I am uncertain if it will make the second edition, but I think this provides more balanced language on left turn options. Most traffic engineers will not endorse split phasing because it is often inefficient and it may require two separate pedestrian timing intervals and generally higher cycle lengths.

Here's some thoughts.

Phase 1 (5-15 Seconds) Protected Left Turns

Left turns protected at the intersection are separated in time from pedestrians and related conflicting through traffic (including bicycle traffic).

Phase 2 (20-40 Seconds)

People walking require sufficient WALK and pedestrian clearance (Flashing DON’T WALK) time to cross the street. WALK time should be maximized where possible to reduce the potential for delay. Right-turning traffic is given the green light concurrent with pedestrians. Turning traffic yields to pedestrians in the crosswalk. A leading pedestrian interval can be used to help people establish their position in the intersection prior to the start of vehicle green.

Restricting Left Turns Phase 3

Restricting left turns eliminates the conflict between left turns and pedestrians. This treatment may shift the conflict to a more desirable location. Left turns have been successfully implemented by time of day in many cities (Reference: http://ops.fhwa.dot.gov/publications/fhwahop08024/chapter4.htm#4.3).  

Application

Protected left turn signal design has multiple warrants throughout the literature, primarily focused on automobile volumes. The need to address safety has recently been evaluated, but still primarily focused on reducing crashes on highways (Reference: http://onlinepubs.trb.org/onlinepubs/nchrp/nchrp_rpt_705.pdf). For the urban setting, subtle, variations in these principles are necessary to prioritize multimodal safety and efficient movement of people and goods. In general, separation of either all movements on opposite approaches, or of specific movements, such as left turns improve safety. The separation comes with the tradeoff of longer cycle lengths.

Restricting left turns by time of day or entirely can serve to address the safety of all users on the street and reducing conflicts that can result in traffic congesting a corridor.

Protected phasing is most often applied in the following situations

• Intersections with history of collisions between left-turning and through vehicles, potentially resulting from geometric constraints or heavy conflicting volumes.
• At locations where pedestrians or cyclists may experience risk associated with left turning traffic relying on the acceptance of gaps
• Where a large skew at an intersections poses a serious risk to oncoming vehicles or pedestrians and may be functionally enhanced through separation.

Benefits and Considerations

Protected left-turn phasing can help reduce the overall risk of pedestrian injury and decreases the potential for crashes.

Use of split-phasing may increase the overall signal cycle length, reducing the overall time available for pedestrian crossings and increasing wait times for all movements.

While split-phasing operations bear consideration in certain locations, a city may elect to restrict left turns entirely or for specific portions of the day when risk of collision is most acute, as an alternative.

Exclusive (Restricting Left Turns) Phasing

Eliminating left turns at an intersection is most often applied in the following situations

• Intersections in a grid network of streets with adjacent intersections that may be better able to handle additional phases in the signalized intersection (depending on block length).
• At locations where pedestrians or cyclists may experience risk associated with left turning traffic.
• Intersections where crossing traffic is significantly high and person carrying capacity may be a higher priority than local automobile accessibility.

Benefits

Eliminating the left turn movement at a particular location reduces the total lost time for the intersection, thus increasing the person carrying capacity of the intersection and reduces the overall cycle length.

Elimination of the left turn also reduces the conflict between through moving traffic and left turns, resulting in an improvement in safety.