The speed-flow curve for freeways has received sufficient study over the past half-dozen years that the representation of it in the US Highway Capacity Manual is undergoing a change. The revised version shows speeds remaining level out to some value of flow that serves as a breakpoint, at which speeds begin to decrease with increasing flow. This paper draws upon work done in the United Kingdom and Germany in an effort to help resolve some remaining issues about this curve. The results provide strong support for the idea that the value of flow at the breakpoint is lower for higher free-flow speeds. The results also suggest that a simple linear function for flows above the breakpoint is adequate for predicting speeds. The most likely value of the slope is -27 km/h for each additional 1000 vehicles per hour per lane. The available data are not adequate to resolve the value of capacity, nor of speeds at capacity. (A)
Abstract