Reliability Analysis of Horizontal Curves on Two Lane Highways

Current highway geometric design practices are based on the concept of design speed and consider all the variables as deterministic. But behaviour of road users is unpredictable and there is always an element of uncertainty in the behaviour of drivers. It is this stochastic behaviour of different road users that necessitated the introduction of reliability concepts into the field of highway design and analysis. In the case of highways, failure is not only represented by crashes, but also the conditions that may force the drivers to behave differently from their original manoeuvres. One of the reasons that force the driver to do so is the deficiency in geometric design. In this study, the reliability analysis was carried out for single horizontal curves using the data of 118 horizontal curves in Kerala. Analysis was done using sight distance, superelevation and extra widening as the geometric characteristics that affects the safety of a horizontal curve. Geometric data, speed data and crash data were used for analysis. Reliability index was calculated for each of the geometric design parameters. A safety evaluation criterion was developed for horizontal curves with the reliability indices as a measure of reliability. EPDO was taken as the safety measure. The threshold levels were identified from the scatter plots prepared between EPDO and reliability indices. Safety Performance Function for horizontal curves was also developed in terms of reliability indices.