Capacity at unsignalized two-stage priority intersections

The subject of this paper is the capacity of minor-street traffic movements across major divided four-lane roadways (also other roads with two separate carriageways) at unsignalized intersections. The center of the intersection, corresponding to the width of the median, often provides room for drivers who have crossed the first half of a major road to stop before proceeding across the second major traffic stream. This situation, which is common with multilane major streets, is called two-stage priority. Here the capacity for minor-street through traffic is larger than at intersections without such a central storage space. The additional capacity being provided by these wider intersections cannot be evaluated by conventional capacity calculation models. An analytical theory is presented for the estimation of capacity under two-stage priority conditions. It is based on an approach by Harders although major improvements were necessary to match the results with realistic conditions. In addition to analytical theory, simulations were performed that enable analysis under more realistic conditions. The result is a set of equations that compute the capacity for minor-street through-traffic movement in two-stage priority situations. These equations are complemented by two sets of graphs that enable easy application of the theory in practice.