FREEWAY CAPACITY: DEFINITION AND MEASUREMENT ISSUES. HIGHWAY CAPACITY AND LEVEL OF SERVICE. PROCEEDINGS OF THE INTERNATIONAL SYMPOSIUM ON HIGHWAY CAPACITY, KARLSRUHE, GERMANY, 24-27 JULY 1991

Freeway capacity is an essential ingredient in the planning, design, and operation of freeways in general and urban freeways in particular. Since normal, day-to-day operation of urban freeways is characterized by recurrent congestion, it is desirable for a traffic analyst to be able to predict the times and places where such congestion will occur, the amount of delay involved, and the traffic flows to be expected in bottlenecks. It is important, therefore, that capacity be clearly defined, be measurable, and be capable of being used in modelling and decision-making. In this paper, several different capacity definitions are examined in the light of these requirements. A combination of philosophical arguments, theory, and data are used to examine definitions falling into four categories: maximum flow, specified percentile flow, mean flow and expected maximum flow. Issues explored include the sensitivity of estimates to the size of observation periods, and when and where capacity can be measured. A by-product of this exploration is the provision of insights into the so-called "capacity drop" issue, that is, whether capacity is adversely affected by the presence of recurring congestion.