ACTS

Roadway Friendliness Tool

Treatment 11: Eliminate Permissive Left Turns

Michigan Standard: This treatment is not required.

Where might you find this on your system: You might find this treatment at intersections with high volumes and/or crashes related to left-turn movements.

Recommendation: Older drivers have difficulty judging gaps in opposing through traffic as they prepare to make a left turn. This problem can be countered by operating a left-turn signal as "protected-only." That is, the permissive phase (turn at your own risk) is not offered. Traffic is given a protected turn phase only (green arrow) during which time the opposing through traffic is not allowed to enter the intersection.

Conflicts with other community goals: When left-turn signals are operated as protected-only, left turn drivers face a red light during all other phases, even when the adjacent through traffic is facing green. During off-peak periods, this can frustrate motorists who might otherwise have been able to execute a left turn with caution. During peak periods, this can reduce the number of drivers able to turn left during a given cycle, and thus reduce capacity at an intersection.

Ease of implementation: Re-timing an existing signal to operate as protected-only left turn requires engineering expertise in the planning stage, and electrical expertise in the installation. The engineering calculations are based on a good understanding of traffic flow and left-turn demand at the intersection, which would usually be obtained from a site review lasting from 2-to-8 hours by a technician. Once the traffic demands have been determined, the re-timing calculations can be done in one or two hours. Field work is limited to one hour performed on the controller box at the side of the road by a one-person crew with no special equipment.

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Roadway Friendliness Tool