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.

A variety of guidelines exist that have been developed to indicate conditions where the benefits of a left-turn phase typically outweigh its adverse impact to intersection operation. Many of these guidelines indicate that a left-turn phase can be justified based on consideration of several factors that ultimately tie back to the operational or safety benefits derived. These factors include:

• Left-turn and opposing through volumes
• Number of opposing through lanes
• Cycle length
• Speed of opposing traffic
• Sight distance
• Crash history

The flowchart shown in FHWA’s Traffic Signal Timing Manual Figure 4-11 provides a detailed approach for the determination of whether a left-turn phase is needed and whether the operational mode should be protected or protected-permissive.

Per FHWA’s Traffic Signal Timing Manual, there are five options for the left-turn phasing at an intersection: permissive only, protected only, protected-permissive, split phasing, and prohibited. Phasing can have a significant impact on signal system effectiveness for a number of reasons, including:

• Permissive only left turn operation may reduce delay for the intersection, but may adversely affect intersection safety, because it requires motorists to choose acceptable gaps. Permissive operation is primarily used when traffic is light to moderate and sight distance is adequate.
• Protected only left-turn phases may reduce delay for turning vehicles but are likely to increase overall intersection delay. This type of operation is recognized to provide the safest left-turn operation.
• Protected-permissive left turn phases can offer a good compromise between safety and efficiency but could limit available options to maximize signal progression during coordination unless innovative displays are used. This mode provides for efficient left-turn movement service, often without causing a significant increase in delay to other movements. This mode also tends to provide a relatively safe left-turn operation, provided that adequate sight distance is available and turns during the permissive component can be safely completed.
• Split phasing left turns may be applicable with shared lanes but could increase coordinated cycle length if both split phases are provided a concurrent pedestrian phase. Split phasing avoids the conflict of 10 opposing left turn vehicle paths. Similarly, if the intersection has high left turn and through volumes, it may be necessary to use shared left turn and through lanes to make efficient use of the approach which would also result in split phasing for the approach.
• Prohibited left turns may be used selectively to reduce conflicts at the intersection.

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.