Yesihati Azati, Xuesong Wang*, Xiaohan Yang, Syed Zaier Zaidi, Mohammed Quddus
Abstract: Downhill curved segments of mountainous freeways are high-risk locations due to the combined effects of longitudinal potential energy and lateral centrifugal forces, and speed limit optimization is a key measure for mitigating crash risk on such segments. However, conventional speed limit settings often rely on uniform standards and fail to adequately account for the combined influences of roadway geometry, slope, traffic conditions, and weather, resulting in heterogeneous safety outcomes across different road environments. This study employs a Causal Forest model with explicitly incorporated quarterly time variables, complemented by a Difference-in-Differences (DID) model, to evaluate the safety effectiveness of speed limit optimization on downhill curved segments of the Guidu Freeway. Empirical results indicate that speed limit measures significantly reduce crash risk, with the Causal Forest model estimating an average treatment effect (ATE) of −0.203 (95% confidence interval: −0.213 to −0.193), demonstrating high precision and robustness and outperforming the DID model in estimation stability. Heterogeneity analysis further reveals that the safety benefits are most pronounced on segments with relatively mild horizontal curvature, low curvature variability, and moderate downhill slopes, as well as under moderate traffic volumes (approximately 7,500–9,000 vehicles/day), while higher truck proportions weaken the effectiveness of the measures. In contrast, fog frequency exerts a relatively limited moderating effect, as treatment effects remain negative across different fog conditions. Overall, the findings confirm that speed limit optimization can substantially improve safety on downhill curved segments of mountainous freeways and highlight the importance of accounting for roadway geometry and traffic composition when designing targeted speed management strategies.
引用：Yesihati Azati, Xuesong Wang*, Xiaohan Yang, Syed Zaier Zaidi, Mohammed Quddus. Evaluation of the Safety Improvement Effects and Adaptability of Speed Limit Measures on Downhill Curved Sections of Mountainous Freeways. Accident Analysis & Prevention, Volume 228, April 2026, 108416.