COMPARISON OF SPEED-FLOW RELATIONSHIPS BASED ON DATA FROM NORTH AMERICA, THE UNITED KINGDOM AND GERMANY

The speed-flow curve for freeways has received sufficient study o the past half-dozen years that the representation of it in the US Highway Capacity Manual is undergoing a change. The revised vers shows speeds remaining level out to some value of flow that serve a breakpoint, at which speeds begin to decrease with increasing f This paper draws upon work done in the United Kingdom and Germany 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 abov the breakpoint is adequate for predicting speeds. The mostlikely value of the slope is -27 km/h for each additional 1000 vehicles hour per lane. The available data are not adequate to resolve th value of capacity, nor of speeds at capacity. (a)