Study of Speed-Flow Relationships on Individual Freeway Lanes

Abstract

Many researchers have examined the form of the relationship between speed and flow on freeways. However, these researchers have concentrated on relationships for the freeway as a whole instead of on individual lanes. In this study, the relationship was examined for each of the three lanes at two locations on Highway 401 in metropolitan Toronto. It proved possible to accurately describe the mean speed in each lane with simple linear functions over the range of flows of most practical interest. Cubic functions provided comparable results over a wider range of flows, but it appears unlikely that the very high and very low flows are of sufficient interest to justify the added complexity. When an attempt was made to examine the relationship between speed and flow for the entire roadway, the linear functions were not adequate, but cubic functions performed reasonably well. However, the details of the full roadway curves are quite different from those of the curves described in the 1994 Highway Capacity Manual. In particular, the curves described in the manual are much steeper than the Highway 401 curves at high flows, implying a much more rapid loss of performance as flow approaches capacity than was observed. The full roadway curves are also surprisingly different from the curves for the individual lanes.