Towards a more inclusive and safe design of horizontal curves: Exploring the association between curve features, reliability measures, and safety

Abstract

Reliability analysis has been advocated as a robust methodology to quantify the risk (known as the probability of non-compliance, P_nc) associated with design limitations such as insufficient sight distance on horizontal curves. This risk represents the probability that the current design (e.g., available sight distance) would fail to meet the requirements of the driving population (e.g., required sight distance). Although previous work has quantified the risk and established links between P_nc and safety, P_nc remains a statistical measure that is not informative enough to roadway designers. To overcome this limitation, the impacts of geometric design attributes on the P_nc as well as the direct and indirect (through the impacts on P_nc) impacts of those attributes on safety need to be modelled and understood. To achieve the aforementioned objective, this paper proposes the adoption of Structural Equation Modelling (SEM) to simultaneously model the relationships mentioned above using data collected on horizontal curves in Alberta, Canada. Light Detection And Ranging (LiDAR) data was first used to extract curve information and assess the 3D available sight distance on 244 curved highway segments. Reliability analysis was then used to quantify the P_nc associated with each curved segment. SEM was then used to test the impacts of curve geometric and traffic attributes on P_nc as well as the direct and indirect impacts those variables have on collision frequency. The results showed that P_nc, curve length, and traffic volume all had statistically significant effects on collision frequency. Curve attributes such as the curve's deflection angle and chord length had statistically significant impacts on P_nc. Both curve deflection angle and the chord length were also found to have an indirect influence on collisions, which was mediated through P_nc. The findings emphasize the importance of considering other curve attributes when designing horizontal curves instead of focusing entirely on the curve radius. The findings of this research provide insights into the indirect impacts of curve attributes of horizontal curves on safety. This could help designers consider curve features that have the highest impacts on non-compliance and safety levels.

Keywords: Horizontal curves; Mediation analysis; Path analysis; Reliability analysis; Sight distance; Structural equation modeling.