CAPACITY OF SIGNALIZED INTERSECTIONS

Standardized methods for estimating capacity at signalized intersections have been sought for at least 25 years. The publication by the Bureau of Public Roads of the 1950 Highway Capacity Manual was the first extensive effort made toward this objective. In 1960, W.R. Bellis, of the New Jersey Department of Transportation, developed a more simplified technique. The results of the research discussed in this paper indicate that the techniques in the Highway Research Boards 1965 Highway Capacity Manual yield greater than 20 percent error in estimating the through capacity of the approach to a signalized intersection for at least half the locations studied. When the Bellis technique was used, less than 20 percent of the locations showed this error. Revisions were made to techniques in the 1965 Highway Capacity Manual, and 20 percent error then was found in only 25 percent of the locations studied. It was concluded that measuring the capacity of an approach in the field is preferable to estimating capacity by using the 1965 Highway Capacity Manual. A method of determining capacity from field measurements is described briefly in the paper.