Comparing the speed-flow relationship for motorways with new data from the M6

This paper draws on data collected in March 1993 from the M6 motorway north of Birmingham, England, to discuss the speed-flow relationships used in cost benefit analysis. Piecewise linear functions have been fitted to the M6 data, and are then compared to the relationships in the U.K. Cost Benefit Analysis Manual (COBA9), and the U.S. Highway Capacity Mnaual (HCM). The results provide clear support for the value of 1200v/h/l used in COBA9 as the flow at which speeds start to decrease more rapidly, but in other respects the results differ from values in both COBA9 and the HCM. In comparison with the observed data, COBA9 estimates: a lower speed at a flow of 1200v/h/l; a steeper slope at higher flows; a higher capacity flow; and a lower speed at capacity. Equivalently, HCM estimates: a gentler slope at higher flows; a higher capacity flow; and a higher speed at capacity. Thus the high-flow data fall somewhere between the COBA9 and HCM estimates.