Effects of Inter-Vehicle Interaction on Speed and Lateral Position for Reviewing Free - Flow Condition

Speed and lateral position of free-flowing vehicles are the performance measures of roadway geometric elements. Most of the earlier studies evaluated a threshold headway based free-flow criteria at an observation station and considered the relevant vehicles for estimation of speed and lateral position. However, evaluation of free-flow at one observation station cannot guarantee the same in the entire segment, due to the possible inter-vehicle interactions prior to the observation station. It can cause inappropriate performance evaluation of the roadway geometric element. Moreover, horizontal curve is one of the critical geometric elements of four-lane highways. Hence, in this research, all the possible inter-vehicle interactions of passenger cars prior to a curve center of four-lane divided highways were identified and analyzed for the effect of inter-vehicle interactions on speed and lateral position. The analysis indicates that speed and lateral position are dependent on the location of inter-vehicle interactions prior to a curve center. Vehicles, which did not interact with other vehicles until the curve center, remains uninfluenced and should be considered for geometric evaluation. This finding is useful to identify the relevant free-flowing vehicles and in-turn, improve the evaluation of horizontal curve geometry.