Reliability-based three-dimensional design of horizontal lateral clearance

The sight distance on horizontal curves could be restricted by a lateral obstruction such as a building or an upward side slope. In current design guides, the minimum horizontal sight offset (HSO) between such an obstruction and the centerline of the inner lane is determined 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, limitations of the current design approach are highlighted. Furthermore, a new reliability-based methodology was developed to calculate HSO on 3D alignments considering the variations of several design parameters. Two design aids were further prepared as an example to show the applicability of the developed approach. The first one is valid for design purposes using 400 m horizontal curve combined with vertical curves of different curvature values whereas the second is valid for hazard assessment of existing designs using 400 m radius and 2H:1V side slope.