Adjusting for the Effect of Bus Blockage on Saturation Flows

Y. A. Tuffour, A.A. Obiri-Yeboah, M. Salifu, P. C. Acquah

Abstract: Roadside friction created by the activities of public transport vehicles at bus bays; taxi ranks; and terminals which have been located in the vicinity of most signalised intersections in Ghanaian cities tends to interfere with vehicular discharge at the intersections. Some vehicles exiting the intersection diverge from the existing traffic stream to the bus bays and taxi ranks while others from these facilities attempt to merge with the stream. In this paper; modifications based on passenger car equivalents and bus blockage time appropriate to the local public transport vehicles have been introduced into the empirical expression for adjusting for bus blockage in the HCM 2010 saturation flow model. Traffic flow data was collected at selected signalised intersections with and without roadside friction within the Kumasi Metropolis and the headway method used to estimate the field saturation flow rates. The HCM ideal flow adjustment model; which incorporated the modifications to the bus blockage factor; was then used to estimate saturation flows at the intersections. The results indicated very good agreement between the observed and model-estimated flows although for most of the intersections (73 %) ; the model values were marginally higher than the observed flows. On average; the overestimates/underestimates were of the order of 1 % for none/low; 2 % for medium; and 7 % for high roadside friction areas.

Keywords: Bus Blockage, Saturation Flow, Signalised Intersection, Roadside Friction, Public Transport Vehicle