Description

This technical report describes the conceptual design and mathematical formulation of the road safety and crash prediction module developed for HDM-4 under the ISOHDM initiative. The paper reviews international literature on road accident causality and establishes macroscopic models that link crash frequency and severity to road infrastructure characteristics, traffic operational factors, and environmental conditions. Key risk factors incorporated into the predictive equations include carriageway width, shoulder width and surface type, horizontal curvature, vertical grade, roadside hazard density, pavement friction/skid resistance, speed variability, and non-motorized transport volumes. The document outlines methodologies for estimating crash rates per vehicle-kilometer across distinct road functional classes in both urban and rural contexts. Furthermore, the report addresses economic valuation of road crashes, detailing methods for monetizing human casualty costs (fatalities, severe injuries, minor injuries) and material damage costs using gross output and willingness-to-pay approaches. The resulting accident model empowers road authorities to quantify safety benefits when evaluating road widening, alignment realignments, shoulder paving, skid resistance restoration, and safety engineering interventions within overall HDM-4 highway investment appraisals. Overall, the report emphasizes rigorous empirical validation, standardized data acquisition protocols, and modular integration into the broader Highway Development and Management analytical engine. By combining physical principles with statistical calibration across international trial sites, this working paper provides a robust theoretical and practical baseline for highway authorities to evaluate long-term infrastructure policy options, allocate maintenance budgets efficiently, and maximize economic returns across regional and national road networks. In summary, this study provides highway engineers, transport planners, and economic analysts with