Mehdi Azimi

Document Type


Date of Award



College of Science, Engineering, and Technology (COSET)

Degree Name

MS in Transportation Planning & Management

First Advisor

Assistant Professor Yi Qi


Left-tum lanes can improve safety and operation of intersections by providing space for deceleration and storage of vehicles waiting to make a left tum. The length of the lane is critical in the design of left-tum lanes. Insufficient length may result in lefttum lane overflow which will seriously impact the safety of the intersection. Therefore it is important to design left-tum lanes with adequate length. The objective of this research is to determine the safety impact of increasing storage lengths of left-tum lanes in intersections. Inadequate length of a left-tum lane results in overflow problem and increases rear-end accident risk in the intersection. In addition, traffic flow will be improved by extending left-tum lanes and eliminating conflicts between through and left-tum vehicles in the lane overflow condition. To achieve the objective of this research, field study was conducted at the intersections with different geometric, signal timing, and traffic flow conditions. Lefttum lane overflow rates were calculated at intersections based on observations in the field. Accident reports related to the study intersections were obtained from traffic management centers. Two approaches were employed to determine the safety benefits of 1 2 increased storage lengths of left-tum lanes: (1) Accident Data Analysis, and (2) Simulation-Based Safety Analysis. It was concluded that extending the length of left-tum lane to eliminate the left-tum lane overflow problem can significantly reduce the risk of rear-end accidents.