Speed-flow Models for Freeways

The new HCM 2010 proposes in chapter 11 two-stage curves to represent the speed-flow diagram for non-congested flow on freeways where constant speeds are postulated over a rather wide range of volumes. This division of the traffic flow in two parts plus the congested conditions is a rather pragmatic approach. Further the three steps pretend three separate traffic flow regimes, which don’t exist. A continuous function, like it is used in the other guidelines, would replicate the traffic conditions comprehensibly and realistically. Therefore, a model based on simple queueing theory analogy is presented to represent the noncongested part of traffic flow. Moreover several models which represent each state of traffic flow by one function are discussed. These different approaches are compared to real world data from the US and Germany. Based on this analysis a new single-stage model for the approximation of fundamental diagrams for freeways is recommended. The model can also be modified in order to represent the capacity drop effect.