ESTIMATION OF FREE FLOW SPEED AND ITS INFLUENCE ON CAPACITY OF TWO LANE ROADS

Abstract

Free flow speed is the desirable speed at which driver tends to move under prevailing conditions such that no preceding vehicles influences the driver's speed. It is to be determined correctly as it plays a vital role in performance evaluation, operational analysis and planning of various facilities. While operating speed is the speed of the vehicles travelling in free flow condition under the favorable weather and prevailing traffic condition. Mostly, 85 1h percentile of free flow speed is considered as operating speed. Many agencies have used as 'a rule of thumb' to obtain the free flow speed by just adding certain amount to the posted speed limit. Actually, the free flow speed or the operating speed on an Inter-urban highway depends on several factors like road roughness, vertical alignment, horizontal alignment, traffic conditions, environmental conditions etc. This report presents a relation between operating speed and the factors affecting it namely road roughness, gradient and curvature and lastly their influence on the capacity of two lane Inter-urban roads. Videos were recorded on nine different base sections of two lane roads to develop a relation between operating speed and capacity of two lane roads. As the objective of the study was to measure the effect of above mentioned factors on operating speed and capacity, the sites were selected in such a manner to quantify the effect of these factors on operating speed. The road and geometric conditions were measured with the help of ARSS Vehicle and ARRB Roughometer and the spot speed data of free flowing vehicles were measured using speed guns. Cumulative frequency distribution curves were drawn to evaluate operating speed. The results show that the effect of road roughness on operating speed is marginal up to IRI value of 4 rn/km and becomes substantial as the roughness increase. The negative effect of curvature and gradient on the operating speed is also observed. The linear regression models were developed to predict the operating speed at any two lane section. The multiple linear regression models were also developed to show the influence of the considered factors on capacity of two lane Inter-urban roads.