Simulation Analysis of Connected Vehicle Signalized Left Turn Assist Technology Shows Up to a 58.4 Percent Reduction in Left Turn Delays and Reduced Crossing Conflicts.

Study Assessed Signalized Left Turn Assist Impacts at an Intersection Using Microsimulation and Surrogate Safety Assessment Models.

Date Posted

Benefits of connected vehicle signalized left-turn assist: Simulation-based study

Summary Information

Despite advancements in traffic engineering, left turns which require drivers to cross oncoming traffic continue to be a major cause of crashes. Permissive left turns, which allow drivers to turn left after yielding to oncoming traffic at signalized intersections, can reduce intersection delays but can be a particularly problematic type of left turn, because they require drivers to make quick judgements about gaps in oncoming traffic. 

One way to potentially improve left turn safety is through the use of Signalized Left Turn Assist (STLA), which is a Connected Vehicle (CV) application. SLTA helps drivers make decisions on when to turn left by monitoring the locations of nearby vehicles, computing gap distances, and informing drivers based on this information.


Researchers used modeling techniques to assess the potential benefits of SLTA. To calibrate a simulation model, researchers utilized video data collected from five intersections in the San Francisco, California area, and processed the data to calculate what drivers consider an acceptable gap for left turns. A case study intersection was used in the calibrated microscopic simulation model to evaluate the intersection with and without CV SLTA using a Surrogate Safety Assessment Model. Model scenarios considered varying utilization rates, which was defined as the CV market penetration multiplied by the assumed driver compliance rate with the SLTA recommendation.


  • SLTA increased left turn capacity  at the simulated intersection by 35.9 to 64.8 percent, depending on the acceptable gap time input parameter, which was evaluated over a range from a high of 5 seconds to a low of 3 seconds.
  • With 100 percent utilization, SLTA reduced left turn delays at the intersection by approximately 58.4 percent
  • Using a gap time of 5 seconds, SLTA reduced the total number of crossing conflicts from six conflicts per hour with no SLTA utilization, to zero conflicts per hour as SLTA utilization rates reached 100 percent.
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