Traffic Capacities on Streets and Highways

There is little doubt that those of us who are involved in the great industry of motor vehicle transportation—whether it be along the lines of design, maintenance, or operations—have come to recognize that street and highway capacity considerations are indispensable. It is unfortunate, however, that too often there is a lack of clear understanding of what we mean by capacity and how it actually is used. I am quite certain that many of us have, at one time or another, used the term capacity rather loosely without fully realizing just what phase of capacity we have meant to describe. Hence, our first approach to the subject should be a definition of terms. Simply stated, capacity is a generic term applied to the ability of a highway facility to accom­ modate traffic. This ability, in turn, is dependent upon prevailing conditions which may be either certain physical conditions, a traffic condition or perhaps both. The committee of highway capacity of the highway research board has fixed upon three terms: basic capacity, possible capacity, and prac­ tical capacity. The first (or basic capacity) is defined as the maximum number of passenger vehicles that can pass a given point during one hour under the most nearly ideal roadway and traffic conditions. An example of this might be the closely controlled flow in the Holland Tunnel or on the Oakland Bay Bridge. Possible capacity differs from the basic level in that it is described as the maximum number of vehicles that can pass a given point in one hour under prevailing roadway and traffic conditions. The third level and the one we will deal with at this time is prac­ tical capacity. This is defined as the maximum number of vehicles that can pass a given point in one hour without causing unreasonable delay, hazard, or a restriction of the drivers’ freedom to maneuver under prevailing conditions.