Investigation of an alternative interpretation of the speed-flow relationship for U.K. motorways

Empirical studies completed in the past half-dozen years in North America have led to a different perspective on the shape of the speed-flow curve. That work was summarized by Hall, Hurdle and Banks, along with a generalised proposal for the shape of the speed-flow curve. The COBA9 speed-flow curve in use in the U.K. differs in several Key respects from the curve proposed by Hall, et al. This paper addresses the question of whether there are U.K. motorway data that would contradict the Hall curve. Initially, the rationale for and interpretation of the Hall curve is summarized. In addition, the difficulties with the underpinnings for earlier North American curves, as represented by the 1965 and 1985 Highway Capacity Manuals are indicated. Potential differences in the determination of speeds are addressed in order to make meaningful comparisons between North American and U.K. speed-flow data. Finally, the studies that form the empirical basis for the COBA9 motorway speed-flow curve are scrutinized to see whether the data in those studies contradict or could equally well support curves of the type proposed by Hall, et al.