Exploration of Vehicle Impact Speed – Injury Severity Relationships for Application in Safer Road Design

The Safe System vision (known as Vision Zero, or Sustainable Safety in Europe) aspires to minimise death and serious injury as an outcome of road travel for all road users. The road engineering contribution to this vision focusses on impact elimination, or where this is not possible, on limiting kinetic energy passed to a road user in case of an impact. Impact kinetic energy is typically limited through vehicle speed management (e.g. speed limits, enforcement or geometric road design), as vehicle mass is difficult to effectively control by public road agencies. This paper reviews available international research on relationships between impact velocity change (delta-v), impact speeds and the probability of fatal and serious injury (MAIS3+) across a range of common crash scenarios. Based on the best identified relationships, and with assumption of equal mass and vehicle speeds, right--angle and head-on impact speeds of approximately 30 km/h are likely to produce MASI3+ injury probability of approximately 10%, considered a critical threshold in Safe System discussions. For pedestrians, the threshold is likely to be even lower, approximately 20 km/h. In rear-end crashes, the critical impact speed was found to be approximately 55 km/h. Some practical applications of these findings are discussed. The paper discusses the assumptions and limitations behind the available evidence, and outlines the proposed research to strengthen it.