OVERTAKING LANES AND THE DISTURBANCE RATE OF THE TRAFFIC FLOW

During two Friday afternoons, over five hour periods, vehicle travel speeds were measured on two passing lanes, 1.0 and 1.8 km in length, and on the intermediate stretch of a two-lane main road, 4.6 km in length. The data analysis, including 2500 vehicles each day, first showed that about five times more passing activities were performed per hour and kilometer on the lanes than on the stretch between. The results also showed that the passing lane of 1 km in length is too short; longer lanes are required for discharging the passing demand of the constrained flow. In this study a simple measure was suggested for the disturbance of the flow. In a given transverse section of the road it indicates any disturbances upstream whether due to road or traffic properties, as related to the optimal, free flow at each volume. It is a percentage measure with the zero value indicating fully free conditions and the unit or 100-% value indicating the condition in which vehicles are staying or crawling at a very slow speed. (TRRL)