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Reliability based highway geometric design Richl, Laurel Anne

Abstract

Geometric design of roads and highways involves the calculation of the minimum stopping sight distance (SSD), minimum length of vertical curves and the minimum horizontal radius. Currently in the Transportation Association of Canada (TAC) design guidelines and in design guidelines published by similar associations, the variables to calculate each of the above listed design parameters are specified. For over ten years, the introduction of reliability theory into geometric design has been investigated (Navin, 1990 and 1992, Easa, 2000). This process has followed the development of limit states design in structural engineering. This research investigated the probabilities of non-compliance with the design guidelines for stopping sight distance, vertical curve length and horizontal curves. Two scenarios were developed for the application of SSD; the first was an operational condition where vehicles should be able to stop before hitting an object on the road. The second scenario was for injury prevention where vehicles should be able to decelerate to a speed at which the probability of a serious injury to a pedestrian or vehicle occupant is low. Reliability methods were used to develop probabilities of a vehicle being unable to stop within a specified SSD on a level tangent, on a curve and on a downgrade. The injury prevention condition was applied to a level tangent whereas the operational condition was applied to all SSD conditions. Similar to the SSD application, reliability methods were used to calculate the probability of non-compliance for the length of crest and sag vertical curves. For this analysis, the stopping sight distance was used as the sight distance criterion. The horizontal curve application investigated two scenarios. The first scenario calculated the probability of a passenger vehicle travelling around a horizontal curve without skidding-out. The second investigated the probability of a vehicle occupant feeling uncomfortable while travelling around a horizontal curve. Each analysis used RELAN (Foschi et. al., 2002), software developed at the University of British Columbia. The probability of non-compliance, beta and the design point were calculated for each design parameter. From these analyses, conclusions were made regarding possible applications of reliability theory to geometric design of roads.

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