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UBC Theses and Dissertations

Effect of the combination of horizontal and vertical alignments on road safety Hanno, Dalal


Current design practices are based on design guidelines considering the road alignment in two dimensions only. Those guidelines were developed without considering the 3 D effect of the combined road alignment. As a result, the design process is oversimplified and does not ensure safety. The objective of this research is to investigate the effect that vertical curves have on horizontal curves and to assess the significance of different road design variables individually or in combination on collision occurrences. In addition, the effect of crest or sag curves on horizontal alignment and overall road safety is studied. As a quantitative measure of the combined effect of road alignment on safety is required, several collision prediction models are developed. The study is conducted using the data of the Trans-Canada Highway for the section between Cache Creek and the Rockies. The Generalized Linear Regression Model (GLIM) was used for finding the significance of various variables and in developing 15 collision prediction models for different combination of horizontal and vertical alignments cases. The results of the study show that the most significant variables found were the exposure variables (traffic and horizontal curve length). Other significant variables found are the value of the horizontal curve radius, the algebraic difference in vertical gradients, the percentage of overlap between the vertical and horizontal curves, the location of the vertical intersection point with respect to the horizontal curve and the ratio between the horizontal curve radius and the vertical curve radius. Some of those variables have a positive relationship with collisions, such as the traffic volume, the curve length and the algebraic difference, and some have a negative relationship, such as the horizontal curve radius, the percentage of overlap, and the distance of the vertical intersection point with respect to the horizontal curve. It has also been concluded that horizontal curves overlapping with crest curves are more prone to collisions than those overlapping with sag curves. Therefore, it is advised to avoid combining minimum horizontal curves with crest curves.

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