Lei Guo

Document Type


Date of Award



College of Science, Engineering, and Technology (COSET)

Degree Name

MS in Transportation Planning & Management

First Advisor

Yi Qi


Left-tum signal operation is a critical component for the safe and efficient operation of a signalized intersection. Left-tum signal control mode and phasing sequence are two major aspects of the left-tum signal phasing design. Selecting proper types of left-tum control mode and sequence will result in reduced intersection delay, improved traffic progress, as well as decreased crash rates. This research attempts to achieve two goals: 1) determine the safety performances of different types of left-tum phasing, i.e., different types of left-tum control modes and sequences and 2) assess the safety benefits of using consistent left-tum phasing along a corridor. To achieve the first goal of this research, data collection was conducted for a wide range of intersections with different left-tum phasing types, geometric features and traffic flow conditions. Then two individual studies were conducted by analyzing the historical crash data collected from the selected intersections: 1) cross-sectional comparison study and 2) before-after comparison study. It was concluded that 1) Protected-only (PO) is the safest control mode, followed by Permissive-only (Perm) and Protected/Permissive (PPLT), 2) Under PO control mode, Lead-Lag is the safest sequence, followed by Lead- 2 Lead and Lag-Lag sequences and 3) Under PPLT control mode, Lead-Lead sequence is safer than Lead-Lag sequence in the condition that the maximum of left-tum volume (of two approaches) is less than 150 vehicle/hour and vice versa. For the second goal, i.e., to assess the safety benefits of using consistent left-tum phasing along corridors, four independent corridor sections with different mix levels of left-tum signal phasing were selected and then the average intersection crash rates on these corridor sections were analyzed and compared. The results showed that the application of mixed left-tum phasing would increase the risk of crashes at intersections