Safety Evaluation of Combined Horizontal and Vertical Alignments on Mountainous Freeways

A safety evaluation method was established based on the driving simulator for evaluating combined horizontal and vertical alignments on mountainous freeways quantitatively. Based on the exact road design parameters from the design blueprint of Yongji freeway in Hunan Province, the driving scene was set up in Tongji University's eight-degree-of-freedom driving simulator. The research objects were upslope+horizontal curve combined alignment, downslope + horizontal curve combined alignment, crest vertical curve + horizontal curve combined alignment and sag vertical curve + horizontal curve combined alignment. With selected evaluation index, multiple linear regression was adopted to develop safety evaluation models based on the driving simulation data. Results indicate that the lateral acceleration is positively correlated with the reciprocal of the radius in the model of all combined alignments. In the model of upslope + horizontal curve combined alignment, lateral acceleration is negatively correlated with maximum grade, and the lateral acceleration is positively correlated with curve length. In the model of downslope + horizontal curve combined alignment, the lateral acceleration is negatively correlated with maximum grade. The 50th/85th percentile values of lateral accelerations were found as the safety thresholds of "GOOD", "FAIR" and "POOR" according to the distribution of them.