Left-turn signals are used at specified signalized intersections. They provide safer and more operationally efficient conditions where there is a high demand for left-turn movements and where there are many challenges associated with executing the left turn.
Fully Protected Left-Turn Phasing
At these signals, there is initially a green arrow, followed by an amber arrow, then a red ball. On the green arrow, drivers are given the right-of-way to complete left turns free of any other traffic or pedestrian conflicts. The amber arrow warns drivers that the left-turn phase is ending. Once the red ball is displayed on the signal heads identified with “Left-Turn Signals” signage, motorists waiting to execute the left-turn movement must wait for the green arrow display during the subsequent cycle. These arrows are helpful in providing a safe opportunity for left-turn motorists to execute their turn, but when there is no opposing traffic, they can cause unnecessary delays.
Protected/Permissive Left-Turn Phasing
Under this arrangement, left-turn signals provide the green arrow, followed by the amber arrow, then a red ball, then a green ball. At some point shortly after the amber arrow is terminated, and the red ball is displayed, drivers will be provided a solid green ball signal. During the display of the solid green ball, left turns can be made when there are adequate gaps in opposing traffic to complete the left turn safely. This type of left-turn phasing is designed to minimize delay by allowing vehicles to turn on the green ball in the absence of opposing traffic. However, the signal still provides a left-turn green arrow when left-turning traffic volume is heavy to help minimize the delay experienced by left-turning motorists. This technique is an efficiency concept (as opposed to an accident reduction concept) since it cannot provide the same degree of safety as an exclusive protected left-turn signal phase.
Criteria for a Left-Turn Signal
The City is using both protected/permissive and protected left-turn signals. Examples of protected/permissive installations include the intersections of Yonge Street and Front Street and Bay Street and Bloor Street West. An example of protected-only left-turn signals is at the intersection of Spadina Avenue and Front Street West.
Criteria for left-turn phasing include vehicle volumes, traffic delay, and collision experience. Any time allocated to left-turn arrow operation must be taken away from the green time allocated to through traffic movements and can thus produce unwanted additional delays for other motorists and pedestrians. Therefore, left-turn arrows are not suitable for every location and, where they are installed, the left-turn signal timing has to fit into the coordination of adjacent traffic signals.
The City favours installing left-turn signals, when signals are upgraded or when new signals are added, provided that there are proven benefits.
Left-Turn Green Arrow Time
The minimum duration of a left-turn green arrow (LTGA) is six seconds. The LTGA is followed by a combined amber/all-red vehicle clearance of four seconds. The six seconds of LTGA provides enough time for at least three vehicles to turn left. An additional two vehicles can turn left when the signals change from one direction (e.g. north-south) to another direction (e.g. east-west). Requests from the public for additional time for a LTGA are reviewed by our District office on an intersection by intersection basis.
The development of signal timing is a “balancing act” where we try to allocate green time fairly to all directions. Any additional time provided to an exclusive left-turn movement must be taken from one of the other movements. In optimising the signal timings, we balance the needs of pedestrians, motorists and public transit by considering the following:
- providing sufficient time for pedestrians to cross the roadway safely
- minimizing the waiting time for pedestrians
- minimizing stops and delays experienced by all vehicles entering the intersection
- accommodating the movement of transit vehicles to provide an efficient and reliable transit system
In some cases, we would prefer a longer LTGA, but are often constrained by high traffic volumes, arriving at the intersection from other directions. For instance, if we add more time to LGTA, we must remove green time from other traffic movements. This often increases delays at the intersection and reduces the overall level of service for other movements. Although adjustment to signal timing and the introduction of LGTAs are some of the measures we use to improve traffic flow at an intersection, implementing such measures do have limitations.