On horizontal curves, drivers may have their available sight distances restricted by a lateral obstruction such as a building or side slope. The minimum horizontal sight offset (HSO) between such obstruction and the centerline of the inner lane is determined in current design guides using the analytic geometry of the sector to relate the curve radius to HSO, given a specific arc length (stopping sight distance). In this paper, the drawbacks and limitations of this approach are highlighted. In addition, a new reliability-based methodology was developed to calculate minimum offsets using a certain probability of hazard (POH). The POH would represent the percentage of drivers who may suffer insufficient sight distance. Moreover, themethodology considers the three-dimensional alignments taking into account the influence of the vertical curvature on the available sight distance.Possible variations of six design parameters are also considered including speed, perception and reaction time, friction coefficient, deceleration rate, and driver eye and object heights. Furthermore, two design aids wereprepared to show the applicability of the developed approach. The first one is valid for design purposes using 400 m horizontal curve combined withvertical curves of different curvature values whereas the second is validfor hazard assessment of existing designs using 400-m radius and 2H:1V side slope.
Abstract