Effect of Horizontal Alignment Characteristics on Capacity Loss at Rural Two-Lane Roads

Abstract

The primary objective of this paper is to explore the impact of horizontal alignment characteristics on capacity and capacity loss at two successive elements, tangent and succeeding horizontal curve. The horizontal alignment chara cteristics include lane width, shoulder width, carriageway width, forward visibility, number of side access points, curve radius, curve length, tangent length, deflection angle and superelevation. Traffic volumes, speeds and densities are used to estimate capacity by extrapolating free-flow observations. Vehicle speeds and flows on the two successive elements were recorded for each five-minute intervals during the survey period, enabling the capacities of the two elements to be estimated and capacity loss d etermined. The effect of different types of vehicle within the traffic stream is normally accommodated for by converting vehicles into passenger car units (PCU) using PCU values. Correlation and regression analyses are used to investigate the relationships between horizontal alignment characteristics and capacity and capacity loss. Several models are introduced to indicate these relationships and the best one was chosen for each case (i.e. capacity at tangents, capacity at curves and capacity loss between t he two successive elements).