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Probabilistic Analysis of Highway Capacity Manual Delay for Signalized Intersections

“Uncertainty analysis” is a new term in traffic capacity analysis, but it is routine in other areas of civil engineering. A probabilistic method, based on uncertainty and sensitivity analysis methods, to incorporate uncertainty of inputs into the existing Highway Capacity Manual deterministic delay model is proposed. Uncertainty analysis is the core of the probabilistic method for propagating the uncertainty of inputs into the uncertainty of the output. Sensitivity analysis is a supplement to uncertainty analysis to guide the data collection, refine uncertainty analysis, and help explain the results of uncertainty analysis. Results of the probabilistic method provide users with a comprehensive view of the level of service (LOS) of a signalized intersection that is more realistic and robust than that of the current method. A case study demonstrates the application of the proposed probabilistic method. With the proposed method, outputs change from a single delay value and LOS letter to mean value, standard deviation, confidence interval, and probability density function for the delay and likelihood percentages for each applicable LOS category such as C (10%), D (76%), and E (14%). This paper also initiates a summary of available field data on uncertain input values for capacity analysis of signalized intersections. A value of 0.05 is likely appropriate for the coefficient of variation for saturation flow, traffic volume, peak hour factor, and green. A value of 0.03 is likely appropriate for cycle length. The two arrival types next to the most probable one should be considered for determining the progression factor.