From severity index and trauma index to severity factor: Development of an improved roadside hazard severity indicator based on physical crash tests for safety barriers

The existing roadside hazard indicator models used in Australasia and most other countries, such as the severity index and trauma index, are based on decades-old data and information primarily sourced from the United States of America. This study is intended to explore a new severity indicator model using safety barrier crash testing data as a more universal data source due to its uniform testing conditions. Five MASH‑tested W‑Beam, five Thrie‑Beam, and seven flexible cable safety barrier products were selected to examine their resulting Acceleration Severity Index (ASI) values measured from physical crash tests. ASI can be treated as an appropriate factor to represent the inherent attributes of a specific safety barrier that affect crash severity. Only tests with similar scenarios (i.e., MASH Test 10) were used to generate typical ASI values assigned to different permanent safety barrier types. A recommended typical ASI value for a selected safety barrier type is combined with a typical ASI value for a tree lined edge as introduced in the existing literature, to form a linear equation. Real‑world crash data in Queensland were used to validate the ASI‑based approach. The ASI values for other roadside objects are derived from this linear relationship and then normalized to form a new model of roadside hazard severity indicator. According to the safety barrier testing results, the recommended ASI values for TL‑3 W‑Beam, TL‑4 Thrie‑Beam, TL‑3 flexible cable, and TL‑4 flexible cable barrier types are 0.72, 0.80, 0.59, and 0.54, respectively, which represent crash severity levels associated with no or minor injury. The validation results present that the error rates do not exceed 22%, demonstrating reliable severity prediction using ASI. Analysis of the MASH TL‑3 W‑Beam data shows that the ASI for this type of safety barrier can be approximated as a function of impact speed and barrier flexibility, and this relationship is validated using real‑world crash data from Queensland. By synthesizing the study data with other published sources, the ASI values were normalized to develop an improved roadside hazard severity indicator i.e., severity factor, on a scale from 0 to 100.