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Safety threshold quantification for mountainous freeways: a CatBoost-SHAP framework for crash frequency modeling on
Huidan Fu1, Xuesong Wang1, Salvatore Damiano Cafiso2
1College of Transportation, Tongji University, Shanghai 201804, China; The Key Laboratory of Road and Traffic Engineering, Ministry of Education, Shanghai 201804, China.
Accident; Analysis and Prevention
|July 13, 2026
Summary
Mountainous freeway safety is improved by understanding how combined horizontal and vertical curve alignments impact crash risks. This study quantifies these interactions to optimize geometric design for safer roads.
Area of Science:
- Road safety engineering
- Transportation engineering
- Traffic safety
Background:
- Geometric design significantly impacts mountainous freeway safety due to complex terrain.
- Current research lacks empirical validation for design limits and interactive effects of geometric parameters.
- Existing models often mask risk heterogeneity on combined horizontal-vertical alignment sections.
Purpose of the Study:
- To develop a quantitative framework for analyzing crash frequency in relation to combined horizontal and vertical curve alignments.
- To classify geometric alignments into four distinct section types for targeted analysis.
- To identify high-risk thresholds for key geometric parameters on mountainous freeways.
Main Methods:
- Classification of geometric alignments into tangent-sag curve (T-SC), tangent-crest curve (T-CC), curve-sag curve (C-SC), and curve-crest curve (C-CC).
- Development of CatBoost regression models using crash, geometric, and operational data from Chinese mountainous freeways (5228 crashes).
- Application of Shapley Additive exPlanations (SHAP) analysis for risk quantification and threshold identification.
Main Results:
- Specific parameter interactions significantly increase crash frequency within each section type.
- T-SC/T-CC sections show increased risk with short tangents, steep grade changes, and small vertical curve radii.
- C-SC/C-CC sections are riskier with high horizontal radius-to-curve length ratios, large curvatures, steep grade changes, and small vertical curve radii.
- Tangents influence crash frequency more than horizontal curves in T-SC/C-SC sections, while horizontal curves exacerbate risk more than tangents in T-CC/C-CC sections.
Conclusions:
- The study provides quantitative insights into the relationship between geometric design and crash frequency on mountainous freeways.
- Findings offer localized support for optimizing geometric design and safety management strategies.
- Identified high-risk thresholds and parameter interactions aid in enhancing operational safety on challenging terrains.
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