Traffic Signal Phasing Modifications
Purpose
Changes to signal phasing can partially or fully reduce conflicting movements between turning vehicles, vehicles going straight, and/or pedestrian and bicyclist movements. Phasing can also be adjusted to potentially reduce excessive queuing and delays and therefore, could potentially reduce aggressive driving behaviors.
Description
The Manual on Uniform Traffic Control Devices (MUTCD) provides regulations on appropriate use of traffic signals and associated phasing. There are multiple phasing modifications that can be made to improve safety depending on the mix of different users, intersection design, and severity of potential conflicts:
Protected Left-Turn Phasing provides a dedicated left turn phase by separating the turning movement from any other conflicting traffic or pedestrian movement. A protected left turn is controlled with a left turn arrow on the signal display.
Changes to Phase Sequencing involves changing the order in which movements are served. Rearranging phases can help reduce long queues and can also align sequences to promote longer pedestrian WALK intervals. This tool requires an extremely specific set of circumstances to provide benefits.
Split Phasing separates traditionally concurrent phases by approach. In other words, one side of the street is serviced separately at a different time. This type of phasing helps remove left turn conflicts from pedestrian crossing phases but will likely result in increased cycle lengths and queuing and will significantly increase pedestrian delays crossing the mainline street.
Safety Benefits
- Protected phasing and split-phasing reduce conflicts between turning vehicles and other road users.
- Twice per cycle left-turn phasing reduces queue spillback and therefore reduces weaving, sideswipes, rear-end collisions, and can also reduce red-light running violations if implemented correctly.
Street Types and Context
Applicable Street Types
All street types with signalized intersections.
Other Location Guidance
- Signal phasing modifications should be based on safety or corridor needs.
- The MUTCD governs the installation of traffic signals and phasing.
Primary User Groups
Key Implementation Considerations
- Concurrent signal phasing tends to keep cycle lengths low and result in less delay compared to exclusive or split signal phasing. Lower cycle lengths are especially beneficial for minimizing vehicular queuing and pedestrian delays.
- Some phasing modifications cannot be implemented without specific lane configurations and designations.
- Changes to phasing often result in larger corridor level impacts to a street network.
Expected Crash Reduction
Changing from permitted and protected left turn phasing to protected-only left turn phasing can reduce left turn crashes 41-48% (FHWA, 2020).
Cost
Adjustments in signal phasing can be relatively low cost, requiring engineering design and modification of signal equipment. However, on large-scale projects or projects involving older existing equipment, it could require a longer time and cost commitment. Additionally, adding exclusive turn phasing may necessitate new signal equipment configurations, which increases the cost significantly (see Traffic Signals). Traffic signals require maintenance and adjustment annually.
Timeline
Varies, from months to three or more years, depending on complexity of the signal modification.
References