An exponential relationship for the proportion of free vehicles on arterial roads

The proportion of free vehicles on a road link is dependent on the geometry of the road link and the presence of intersections along the link. This paper details the investigation of the proportion of free vehicles as a function of the link geometry. It is necessary to define the effects of the link geometry before the effect of intersections on the headway distribution can be incorporated. Current methods for describing the proportion of free vehicles on a road link predominantly use linear relationships with the vehicle flow rate. An exponential relationship has been examined and found to be better suited to the modelling of the proportion of free vehicles than the existing linear relationships. In modelling the flow in each lane, the characteristics of the traffic flow were found to be dependent on the lane type being modelled. The kerb lane sustained a higher proportion of free vehicles than the median lane for similar flow rates. This finding is significant in the analysis of priority intersections. The difference is attributed to driver behaviour and is related to the expected ease of overtaking. Using these results the proportion of free vehicles is described as a function of the lane type, lane width and the vehicle flow rate through the use of exponential relationships. The influence of intersections upon the characteristics of the traffic flow can then be incorporated by describing the effect of the proportion of free vehicles on the road link.