Risk-Based Highway Design

Existing geometric design guides provide deterministic standards for design requirements primarily based on near worst-case scenarios and conservative percentile selection of design parameters. Probabilistic geometric design analysis has been advocated to represent realistically the randomness in design parameters and variables. Probabilistic techniques can provide a measure of the degree of deviation from design standards. Collision modification factors have been advocated as quantitative measures of the impact on safety associated with changes in road features or traffic control. Often, however, no collision modification factors exist in the literature to predict the safety impact of changing particular road features. An important example is sight distance restriction on horizontal curves. Many highways in British Columbia, Canada, are located in mountainous terrain where the additional cost of earthwork or land acquisition to accommodate lateral road expansion can be prohibitive. In this constrained environment, a typical trade-off arises between design requirements (e.g., adequate sight distance on a horizontal curve) and budgetary constraint. The resolution requires comparing the consequences of every alternative. In these cases, reliability analysis can be used to evaluate the risk of deviating from the design requirements. A decision-support tool was developed to compare the risk of different deviations from sight distance requirements. Two case studies were used to investigate the safety implications of sight distance limitation on road segments, the risk associated with deviation from standards, and risk variations among the road segments. The proposed road design is associated with relatively high risk of limited sight distance, and the risk levels associated with standard design requirements vary significantly.