Left Turn Signal Phasing Guidance
One of the most important factors impacting both the safety and operations at a signalized intersection is signal phasing.
Both nationally, and across Virginia, differing methods have been developed to aid and support left-turn phasing decisions.
In order to support greater consistency in left-turn phasing guidance across the state while maintaining flexibility in left-turn phasing decisions, a team of Virginia Department of Transportation (VDOT) signal professionals developed the left-turn phasing information provided below.
This practice combines policy, guidance and a supporting tool.
On Feb.23, 2015, VDOT's Traffic Engineering Division issued a clarification to TE-362.1 regarding Professional Engineer signing and sealing of an Engineering Assessment for new and changes to left-turn signal phasing.
This clarification specifies:
- New and changed left-turn phasing mode decisions are to be made following completion of an Engineering Assessment that is a formal document signed and sealed by a Professional Engineer
- This Engineering Assessment is independent of traffic signal plans
- There is no specific format for such Engineering Assessment, but identifies a guidance document and documentation workbook are available to support documentation in a clear and consistent manner.
Note: This current clarification and the Engineering Assessment requirement is NOT applicable to right-turn phasing, pedestrian phasing and phase sequencing (ie. lead, lag, lead/lag) decisions.
VDOT's Guidance for Determination and Documentation of Left-Turn Phasing Mode:
- A framework for consistent and comprehensive left-turn phasing decisions
- Supports application of engineering judgment and in many cases encourages or validates that judgment can be applied
- Addresses technical considerations of the most common left-turn phasing factors:
- Sight distance
- Number of turn lanes
- Intersection geometry
- Crossing distance
- Number of opposing lanes
- Critical crossing gaps
- Correctable left-turn crashes
- Pedestrian / multimodal considerations
- Network considerations
As guidance, no “shall statements” are included and there is no requirement mandating its use. A critical facet to the utility of this guidance is the application of engineering judgment, the acceptance that judgment will likely differ amongst professional engineers and the recognition that left-turn phase selection is not a strictly quantitative, objective process.
The Left-Turn Phase Selection Engineering Assessment Workbook may be used to document left-turn phasing Engineering Assessments in a consistent and comprehensive manner.
The workbook works collaboratively with the guidance document to first evaluate the major left-turn phasing factors for each approach and then collectively at the intersection level.
Use of the workbook acts as documentation to support future review and evaluation of past engineering decisions and aids VDOT in review of engineering work prior to approval.
Signal and Arterial Systems Program Manager
Mike Clements, P.E.
Senior Signal Systems Engineer
(Don) Ritchie Robbins, P.E., PTOE