Abstract:The two main approaches to capacity and delay estimation are probabilistic and deterministic. The probabilistic approach is used for designing unsignalized intersections in the United States; the deterministic approach is used for the same purpose in Great Britain. The probabilistic approach given in the 1985 Highway Capacity Manual underestimates capacity. Reevaluating both the probabilistic and deterministic approaches is necessary. A clear evaluation framework is developed for comparing these two approaches from the perspective of modeling theory. The two approaches are evaluated in terms of theory and methodology, validity, policy sensitivity, simplicity, data requirements, and compatibility. The strengths and weaknesses of each approach toward providing future research directions on unsignalized intersectons in the United States are discussed.