Existing geometric design guides provide deterministic standards for design requirements which are mainly based on near worst-case scenarios and conservative percentile selection of design parameters. The selection of thepercentile values is not based on definitive and explicit safety measures. In practice, collision modification factors (CMFs) are advocated as quantitative measures of changes in road features on safety. However, in many situations, there are no CMFs in the literature to predict the safety impact of changing particular road features. An important example of these road features is sight distance restriction on horizontal curves. Many highways in British Columbia are located in mountainous terrain where the additional cost of earth work and/or land acquisition to accommodate lateral road expansion can be prohibitive. In this constrained environment, a typicaltrade-off arises between design requirements (e.g. adequate sight distance on a horizontal curve) and budgetary constraint. The settlement of this trade-off 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. This paper presents a decision support tool to compare the safety impact of different deviations from sight distance requirements. Two case studies of proposed major highway developments inBritish Columbia are presented. Two issues are investigated. First, what are the safety implications of sight distance limitation on road segments in these case studies? Second, how much is the risk associated with deviation from standards and how different is this risk among the road segments in the case studies? The results show that the proposed road design in thecase studies is associated with relatively high risk of limited sight distance. They also show that the risk levels associated with standard designrequirements vary significantly. Measures to mitigate this high risk weresuggested.
Abstract