Contributing factors influencing injury severity in two-vehicle and multi-vehicle crashes: A comprehensive review of driver, vehicle, roadway, environmental, and crash characteristics

Road traffic crashes are an important global safety concern, with injury severity influenced by multiple interacting factors. Unlike prior reviews, this study presents an integrated review of their structural hierarchy and interaction mechanisms for two-vehicle and multi-vehicle crashes. A structured PRISMA-based approach was utilized to analyze 71 peer-reviewed articles (2002-2026). Determinants were categorized as driver, vehicle, roadway, environmental, and crash-related factors. Findings show a clear hierarchy wherein crash- and driver-related factors (speeding, impairment, collision type, and physical condition) are the factors most consistently associated with injury severity outcomes because they directly influence crash dynamics. Vehicle characteristics are moderators; however, roadway and environmental factors are context-bound and can influence severity indirectly. The study emphasizes injury severity as an interaction-driven outcome and indicates the increasing reliance on mixed logit and machine learning models, and calls for more interpretable and interaction-aware models that can be used to improve policy relevance.