Risk-based approach in highway geometric design

In current geometric design guides, the highway elements are designed using the deterministic approach. This approach considers below average driver characteristics and vehicle capabilities to produce designs that would satisfy the majority of road users. However, a design following this approach could be conservative and uneconomic since the probability of having simultaneous critical values for all design parameters could be very low. In addition, the current design methods rely on either horizontal or vertical projection of the road alignment, which may cause either underestimation or overestimation of the road supply.

In this research, a comprehensive approach for geometric design was developed based on reliability theory. The approach was applied to the sight distance design basis using the required sight distance (RqSD) as a demand and the distance actually available to drivers (AvSD) as a supply. Both simulation and analytical reliability techniques were used in the analysis. For RqSD calculations, a new approach was developed to account for the continuous change of longitudinal grade, expected loss of friction on horizontal curves, and it considers both deceleration rate and friction coefficient. As for AvSD calculations, both two and three dimensional values were calculated taking advantage of two existing programs based on the finite element technique.

Using Visual Basic.Net programming language, a computer program was developed; namely sight distance evaluation system (SDES). Using SDES, profiles can be generated for RqSD and/or AvSD at pre-determined intervals. At a given road section, RqSD and AvSD distributions can be evaluated to determine the probability of hazard (POH), which reflects the percentage of drivers who may suffer insufficient sight distance. POH profile was further evaluated in a real case study using data from Highway 61, Ontario. The POH values less than 0.05 % have mainly resulted along the entire route except for a few sections that had a maximum POH of 7.45 %. In addition, a reliability-based approach was developed to locate lateral obstructions along three-dimensional alignments using a certain POH. Two design aids were further developed for design and evaluation purposes on 400-m curves and 2H:1V side slope.