Optimum loop placement that balances operational efficiency and dilemma zone protection

"Report Date: December 2010."; "FHWA/OH-2010/17"--Technical rept. documentation p.; Includes bibliographical references (p. 78-81).; Final report.; Sponsoring agencies: Ohio Dept. of Transportation, Federal Highway Administration; State Job No. 134433Current methods for advance loop placement at high speed approaches in ODOT are based on previous study results, in which the dilemma zone was computed using constant contributing factors. In reality, those contributing factors are varied with different travel behaviors like varied speeds. Accordingly, the dilemma zone is greatly dependent upon the dynamics of those contributing factors. In order to reveal the dynamic features of the dilemma zones, qualified ground-truth trajectory data of 1445 vehicles are extracted using video-capture-based technology from 46-hour high resolution videos at four high-speed signalized intersections in Ohio. As a result, the dynamic dilemma zone models are developed and validated, and then two dilemma zone look-up tables are created to show tabular results in estimating the dilemma zone ranges and locations at different durations of yellow time and speed limits versus different yellow-onset speed intervals, respectively. A VISSIM-based microscopic simulation test bed is established to examine the current ODOT practice of loop placement and results in optimum alternatives of the placement schemes. The evaluation results indicate the ODOT practice reduces the dilemma zone related traffic conflicts by at least 70% at different speed limit conditions. In overall, it is more effective in providing dilemma zone protection than the scenario without advance loops. However, the current method is proved insufficient to enhance the operational efficiency of the intersection in great part. Typical evidence lies in the evaluation findings that the overall intersection delay is greatly increased at the speed limit of 55 mph if the current loop placement method is used. The proposed loops placement alternative outperforms the ODOT's current practice in terms of reducing the dilemma zone related traffic conflicts by at least 84% under different speed limits. It also performs better in terms of operational efficiency at all speed limits, particularly in the case of higher speed limits such as 50 mph and 55 mph