Exploring the Relationship Between Highway Geometric Characteristics and Capacity Loss

Abstract

The main aim of this paper is to explore the impact of highway geometric characteristics on capacity at tangents and horizontal curves as well as on capacity loss at the change from tangent to curve. Vehicle speeds and flows on the two successive elements (tangent and curve) were recorded for each five-minute intervals during the survey period, for twelve rural two-lane road sites. A capacity estimation method based on extrapolation from a fundamental diagram representing the relationship between traffic flow and density was used. The effect of different vehicle types was considered by converting them into equivalent passenger car units. Regression analysis was used to investigate the relationships between geometric characteristics and capacity for tangents, curves and capacity loss between the two elements. The significant independent variables for tangents are lane width, shoulder width and tangent length. In the case of curves, the significant independent variables are curve radius and lane width. The best model that shows the relationship between capacity loss and geometry characteristics includes only curve radius. Based on this model, a curve radius with 600 m appears to be the threshold value to diminish the capacity loss between tangent and succeeding curve. The models are very useful and can be used to deal with capacity analysis and evaluation, especially for the area under study.