Study on the Relationship between the Horizontal Alignment Indices and Traffic Safety in Mountainous Freeway

To solve the problem of safety evaluation in the alignment design of a highway, a horizontal alignment indices-based design consistency method was studied. The key indices of horizontal alignment, such as the ratio of the individual curve radius to the average radius (CRR), curvature change rate (CCR), and the ratio of the individual tangent length to the average tangent length (TLR) are put forward. The relationship between horizontal alignment indices and fatal crashes on XIHAN mountainous freeways is analyzed as a case study. The results shows that there are higher accident rates when CRR ranged from 0.4 to 0.6, with CCR of about 100∼190 gon/km on horizontal curves, and with TLR below 0.9 on tangents. The application on the mountainous freeway indicates that the CRR, CCR, and TLR are key indicators to the degree of traffic safety.