Effect of Highway Geometric Characteristics on Capacity Loss

The estimation of roadway capacity is essential in the planning, designing, and operation of highway facilities. This paper aims at assessing the influence of highway geometric characteristics on capacity at tangents and horizontal curves as well as on capacity loss at the change from tangent to curve. Traffic and geometry data obtained from twelve rural, two-lane road sites in Minoufiya Governorate, Egypt, were used. Each site consists of a tangent element and the succeeding horizontal curve. Vehicle flows and speeds were collected at each element in the study sites. A capacity estimation method that was based on extrapolation from a fundamental diagram which represented the relationship between traffic flow and density was used. The effect of different vehicle types was accommodated for by converting them into equivalent passenger car units. Regression analysis was used to investigate the relationships between geometric characteristics and capacity. The best regression models for each case (i.e. capacity at tangents, capacity at curves, and capacity loss between the two elements) were introduced. For tangents, the significant independent variables are lane width, shoulder width, and tangent length. In the case of curves, the significant variables are curve radius and lane width. The best model that exhibits the relationship between capacity loss and geometry characteristics includes only curve radius, which functions as an independent variable. The models are very useful and can be used to deal with capacity analysis as well as for the evaluation of rural, two-lane roads, especially for the area under study.