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. Piece-wise linear functions have been f itted to the M6 data, add are then compared to the relationships in th e U.K. Cost Benefit Analysis Manual (COBA9: U.K. Department of Transpo rt, 1981), and the U.S. Highway Capacity Manual (HCM: U.S. Transpn Res Ed, 1994). The results provide clear support for the value of 1200 v/ h/l used in COBA9 as the flow at which speeds start to decrease more r apidly, but in other respects the results differ from values in both C OBA9 and the HCM. In comparison with the observed data, COBA9 estimate s: a lower speed at a flow of 1200 v/h/l; a steeper slope at higher fl ows; a higher capacity Row; and a lower speed at capacity. Equivalentl y, HCM estimates: a gentler slope at higher Bows; a higher capacity fl ow; and a higher speed at capacity. Thus the high-flow data fall somew here between the COBA9 and HCM estimates.