| Current horizontal curve geometric design policy, contained in the American Association of State Highway and Transportation Officials’ A Policy on Geometric Design of Highways and Streets, is not performance-based, establishing only minimum criteria and limiting values centered on the design speed concept. The current model used for horizontal curve design considers demand side friction as a passenger comfort level, and does not account for the effects of longitudinal friction demands (such as vertical grade or vehicle deceleration). This research uses a modified version of the point-mass model, with data collected at 99 horizontal curves in 7 states, to develop a probabilistic approach for horizontal curve design. Pavement friction supply and demand are compared using first order reliability methods. A reliability index is established for each of the observed horizontal curves and its association to crash frequency is assessed. Results indicate that trade-offs in design values (such as curve radius or superelevation rate) are quantifiable for a performance based design approach. The results show that vertical grade and driver deceleration should be considered, especially in high-speed design. An alternative design methodology is proposed as part of this research. |
