Speed–Flow Curves for Freeways in Highway Capacity Manual 2010

Since the 1965 edition of the Highway Capacity Manual (HCM), fundamental curves representing the relationship between speed and volume or flow rate have formed the basis of design and analysis methodologies for basic freeway segments and for uninterrupted flow segments of multilane highways. As the 2010 edition of the HCM was being developed, a new set of freeway speed–flow curves was recommended. Somewhat late in the development process, the recommended curves became controversial. This paper presents the recommended curves and the research behind them and the controversies that affected the outcome. A final set of curves for HCM 2010 was recommended on the basis of extensive comparative statistical analyses of a substantial database and on several conditions that the TRB Highway Capacity and Quality of Service Committee voted to impose on the curves characteristics. Recommendations for the future are made.