Analysis of Decision Boundaries for Left Turn Treatments

The Utah Department of Transportation (UDOT) continues to place the safety and operations of state roadways at the forefront of their priorities. At times, however, safety and operations may conflict with each other, especially when related to left-turn phasing. Although it is desirable to provide protected left turn phasing whenever possible to improve the safety of left-turn movements, the time allocated to the protected movement may have a negative impact on through traffic movement. Recent statistics (2015) show that 37% of crashes across the state occur at intersections. Of these, 59% occur at signalized intersections, and 31% of those crashes involved a left-turning vehicle. The topic of left-turn phasing is not new to UDOT and a variety of projects have been conducted recently (or are ongoing) to evaluate safety of left-turns as well as to evaluate the warranting of left-turn signal phasing. Even with this research, there is a need to better understand the safety and operational effects of left-turn signal phasing and to identify boundaries for left-turn treatments at signalized intersections based on actual data from across the state. The purpose of this research is to evaluate the mixture of left-turn and opposing through traffic volumes for permitted vs. protected left-turn phasing at intersections and to identify cut-off points as a result of the data that would help to identify when to switch from permissive to protected phasing at intersections, as well as when to transition from single to dual left-turn facilities. It is anticipated that would be done using the signal performance measurement data combined with the safety crash database to correlate operations and safety. Because of the randomness of crash data and potential limits to the data that are available, it is recommended that the field measured data be supplemented with simulation data using VISSIM and the Surrogate Safety Assessment Model (SSAM) developed by the Federal Highway Administration (FHWA). This will allow the research team to control the left-turn volume and opposing traffic volume levels to aid in determining meaningful boundaries for the different left-turn treatments. Practical guidelines need to be established to evaluate permissive single left-turn lanes, protected/permissive single left-turn lanes, and protected dual left-turn lanes. To meet the purpose of the research, the research team would complete a literature review to gain insight and understanding on existing decision boundaries for left-turn treatments at signalized intersection and to identify how best to utilize the data that are available in Utah to address the needs outlined. Upon completion of the literature review, the data across the state would be analyzed to identify cutoff points that could be used to establish guidelines, while also analyzing the crash profile of left-turning vehicles to aid in this analysis.


    • English


    • Status: Active
    • Funding: $70,000
    • Contract Numbers:


    • Sponsor Organizations:

      Utah Department of Transportation

      4501 South 2700 West
      Project Development
      Salt Lake City, UT  United States  84114-8380
    • Managing Organizations:

      Utah Department of Transportation

      4501 South 2700 West
      Project Development
      Salt Lake City, UT  United States  84114-8380
    • Project Managers:

      Jensen, Travis

    • Performing Organizations:

      Brigham Young University

      Provo, UT  United States  84602-4081
    • Principal Investigators:

      Schultz, Grant

    • Start Date: 20180129
    • Expected Completion Date: 20190430
    • Actual Completion Date: 0

    Subject/Index Terms

    Filing Info

    • Accession Number: 01668382
    • Record Type: Research project
    • Source Agency: Utah Department of Transportation
    • Contract Numbers: 18-8736
    • Files: RiP, STATEDOT
    • Created Date: May 2 2018 2:57PM