Speed - Flow Models for Uninterrupted Traffic Facilities

The report explains the uninterrupted travel speed concept, and describes the HCM speed - flow models for basic freeway segments and multilane highways as well as for the running time component of the HCM travel speed model for urban streets. The HCM speed - flow models are given for undersaturated conditions on these uninterrupted-flow facilities. Multilane highway analysis is qualified as relevant to "rural and suburban" highways. Urban roads with signalized intersections spaced at 3 km or more fall into this category, otherwise they are classified as urban streets. The aaSIDRA (SIDRA INTERSECTION) speed - flow relationships for uninterrupted movements were calibrated against the US Highway Capacity Manual speed - flow models. This effort indicated various unexpected characteristics of the HCM 2000 models. The report presents the results of an investigation of issues of concern with the HCM models. A proposed solution based on the use of "Akçelik’s " speed - flow function is presented. Akçelik's function, which applies to both undersaturated and oversaturated conditions is described in detail.