COMPARING THE SPEED-FLOW RELATIONSHIP FOR MOTORWAYS WITH NEW DATA FROM THE M6
W. Spencer Smith, Fred L. Hall and Frank O. Montgomery
This paper draw on data collected in March 1993 from the M6 motorway north of Birmingham, to discuss the speed-flow curves used in the U.K. Cost Benefit Analysis manual, COBA9. Piece-wise linear functions have been fitted to the M6 data, and are then compared to the curves in COBA9 and the U.S. Highway Capacity Manual (HCM). The results support the value of 1200 v/h/l used in COBA9 as the flow at which speeds start to decrease more rapidly, but in other respects the results differ from the values in both COBA9 and the HCM. The speed estimated by COBA9 at a flow of 1200 v/h/l is lower than that observed; the slope estimated by COBA9 is higher than that observed; and the speed at capacity given in COBA9 is lower than observed. On the other hand, comparison with the HCM shows that the slope it predicts for higher flows is gentler than that observed; HCM capacity flows are higher than those observed; and the HCM speed at capacity is higher than that found from the data. Thus the high-flow data fall somewhere between the COBA9 and HCM curves.