Evidence-based roadside clear-zone design for mitigating run-off-road and rollover crashes on rural highways in Saudi...

Saudi Arabia's rural highway network experiences a high number of severe run-off-road and rollover crashes, yet evidence-based, locally validated roadside clear-zone design guidance remains limited. This study investigates how roadside clear-zone characteristics influence the occurrence and severity of these crashes by integrating statistical analysis of in-depth investigations of multi-fatality crash data with high-fidelity vehicle simulations. An in-depth dataset of 127 multi-fatality crashes (2020-2022) from the National Road Safety Centre was analyzed using binary logistic regression to identify key predictors of run-off-road severity. The final model retained three significant variables: Vehicle Type (odds ratio OR = 0.56 per class increase, p < 0.001), indicating that heavier vehicle categories diminish rollover risk; Posted Speed (OR = 1.017 per km/h increase, p = 0.032), reflecting elevated crash likelihood at higher speed limits; and Steep Slope (V:H ≥ 1:4) (OR = 2.73, p = 0.054), underscoring the destabilizing effect of steep embankments on vehicle stability. Descriptive statistics revealed an average of 3.59 fatalities and 2.80 injuries per crash, with single-vehicle incidents comprising 78% of the sample. Complementary finite-element simulations using AnalyzerPro examined sixteen scenarios that varied clear-zone width (3-7 m), embankment gradient (1:2.5 vs. 1:10), and soil compaction at speeds of 100 km/h and 140 km/h. Widening the clear zone from 3 m to 7 m reduced rollover probability by approximately 45%, while fully compacted embankments and slopes consistently prevented rollovers across all vehicle classes and speed profiles. Steeper slopes markedly increased rollover propensity relative to gentler grades. These findings provide robust, evidence-based guidance for updating roadside design standards-emphasizing clear-zone widening, soil compaction protocols, and slope moderation-to enhance safety on Saudi Arabia's rural road network.