Protected left turn signals
At these signals, there is initially a green arrow, followed by an amber arrow, followed by a red arrow. On the green arrow, drivers are given the right-of-way to complete left turns free of any other traffic conflicts. The amber arrow warns drivers that the left turn signal is ending. On the red arrow, left turns are not permitted. These arrows are helpful, but when there is no opposing traffic they can cause unnecessary delays.
Protected/permissive left turn signals
Under this arrangement, left turn signals provide the usual green arrow which is followed by the amber arrow. After the amber arrow is terminated, drivers are facing 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 eliminating the need for the red arrow and allowing vehicles to turn on the green ball after opposing traffic has cleared. By not having the red arrow, motorists do not have to wait to turn left when there is no opposing traffic, a situation that often occurs during periods of low traffic volumes. However, the signal still provides a green left turn arrow when left-turning traffic is heavy.
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.
Why doesn't the City use left turn signals everywhere?
The City is using both protected/permissive and protected-only left turn signals. Examples of protected/permissive installations are at the intersections of US 285 & Elati, US 285 & Logan, US 285 & Broadway, and at Broadway & Dartmouth. An example of a protected-only left turn signal is at the intersection of Broadway & Belleview.
The Federal Highway Administration, in its Traffic Control Devices Handbook, offers suggested guidelines for left turn phasing. These include vehicle volumes, traffic delay, and accident experience.
Any time used for arrow operation must be taken away from the green time allocated to through traffic movements. Therefore, left-turn arrows are not suitable for every location. And where they are installed, the left turn signal timing should not disrupt the coordination (progression) of nearby traffic signals.
The City favors installing left turn signals, as existing signals are modernized or new signals are added, whenever they provide proven benefits.