Safety and Operations Assessment of Various Left-turn Phasing Strategies

This research evaluates the safety and operational impacts of different left turn movement treatments at signalized intersections. The project (1) compares safety and operational impacts of Protected-Only Left-Turn (POLT) phase against Protected-Permissive Left-Turn (PPLT) phase with a Flashing Yellow Arrow (FYA) indication, (2) compares safety and operational impacts of doghouse display against four-section vertical display for PPLT with a Flashing Yellow Arrow, (3) and identifies whether time-of-day (TOD) variable left-turn control mode with FYA induces confusion among left-turning drivers. Literature shows that converting a POLT to a PPLT control mode is associated with an increase in crash rates while reducing intersection delay. Previous research recommended to select the control mode considering traffic volume, speed limit, sight distance, number of lanes, and crash history. Previous research shows that doghouse displays are associated with larger crash modification factors, more confusion among drivers, and higher delays compared to a four-section vertical display with an FYA. The results of our driver comprehension survey showed that half of the drivers who have encountered intersections with variable left turn control mode by TOD felt confused by such phasing strategy. On the other hand, the simulation-based analysis showed that changing the left turn control mode by TOD yields more efficient traffic operations and lower average delays.

Publication Date: 
Monday, October 15, 2018
Publication Number: 
WA-RD 809.1
Last modified: 
09/17/2019 - 10:45
Authors: 
Ali Hajbabaie, Sattar Sattarov, Rasool Mohebifard
Originator: 
Washington State University. Department of Civil and Environmental Engineering.
Number of Pages: 
116
Subject: 
Left turns, Left turn phase, Phases by type of traffic, Turning traffic, Drivers, Driver performance, Signalization, Traffic signal indications, Safety, Evaluation and assessment, Alternatives analysis, Periods of the day, Arrows (Signals).