SPEEDS AND FLOWS ON AN URBAN FREEWAY: SOME MEASUREMENTS AND A HYPOTHESIS

Speeds and flows were measured in a bottleneck section of a six-lane urban freeway near Toronto, Canada, on three successive mornings. The average capacity flow was 1984 passenger-car units per lane per hour, very close to the traditional value of 2000, but at an average speed of 80 km/h (49 mph), a much higher speed than is usually indicated in textbooks and manuals. Frequency distributions of the observed flows and speeds at capacity are reported and used as a part of a general discussion of the meaning of the term "capacity." In order to study the relationship between speed and flow, measurements were also made before the section reached capacity. At flows less than three-fourths of capacity, the average speed was 100 km/h (62 mph); there was no apparent decrease in speed as the flow increased. Between three-fourths of capacity and capacity, a gradual reduction in speed from 100 km/h to the 80-km/h speed observed at capacity was expected, but no such smooth speed transition was observed. The nature of the data leads to the hypothesis that the average speed on an urban freeway with a speed limit, where neither grades nor curvature is severe and where the traffic is not affected by downstream bottlenecks, may not vary as a function of flow but may depend only on whether the traffic is or is not a capacity flow discharged from an upstream queue. (Author)