Flow Rate Effects on Vehicle Speed at Two Way-Two Lane Rural Roads

The flow characteristics of a cross-section are identified by time headway and vehicle speed distributions over time. Knowledge of both headway and speed distributions plays a significant role in several fields of traffic flow analysis and simulation. In particular, we refer to operative analysis of road facilities in interrupted and uninterrupted flow conditions. This work identifies typical vehicle speed probability density functions (pdf's), for two-way two-lane road segments. The scarcity of empirical findings on these distributions in the literature (particularly for two-way two-lane roads) was one of the reasons for the study. The endogenous traffic parameters considered to affect speed distributions, flow rate and flow composition were taken into account. Mixture models for vehicle speeds were estimated. The data used for statistical analysis of headway and speeds came from traffic observations carried out on the rural road network of the province of Verona (north-east Italy). In the case in question, regardless of traffic conditions, the Skew-T single model turned out to be the one which best fitted the observed phenomena. The effect of flow rate in both directions (analysed and opposite) on vehicle speed distributions was also analysed. As expected, in all traffic conditions, the curves move to the left and become narrower as the flow rate increases. This effect seems to be less pronounced, where we measure the effect of the opposite flow rate.