Left-Turn Phasing—State-of-the-Art Review and the Outlook for Future Improvements

Left-turns are one of the most critical maneuvers at signalized intersections. There are several types of left turn signal phasing in use: protected-only, permitted/protected, permitted-only, and prohibited. If the protected part of a left turn phase is assigned before the through phase starts, a left-turn sequence is called lead. If the opposite is true, it is called lag. Currently there are no uniform guidelines on left-turn installations. Furthermore, the practice on left-turns is not consistent. This paper summarizes a literature on left turn phasing, sequencing and left-turn signal displays (e.g., flashing yellow arrows) and points out the key findings and shortcomings. The authors propose computational and simulation tools—driving simulators, microsimulation, surrogate safety models and augmented reality for future research which could lead toward the unification of left-turn set of guidelines.