Road Design

This report reviews current literature and numerous existing full-scale crash test results to analyse trends in occupant impact measures and identify opportunities for further research into the relationship between these measures and actual injury outcomes.

The relationship between the Manual for Assessing Safety Hardware (MASH) occupant impact measures and the likely injuries is a constant discussion point when evaluating road safety barriers. This report consolidates findings from literature and crash tests to explore this relationship and highlight possible enhancements in the evaluation, design and installation of road safety barriers.

This report builds on the findings of Austroads Project ABT6152, which found that stiffer bridge parapets result in higher vehicle decelerations and may pose higher occupant risk than concrete road safety barriers.

While MASH limits are considered conservative, the extend of this conservatism is not well understood. For this report, vehicle trajectory was quantified using the Acceleration Severity Index (ASI), and likely head injuries were evaluated using the Head Injury Criterion (HIC) measured over 36 milliseconds. A regression equation was fitted to data from barrier impact tests conducted by Crashlab and those recorded in the literature.

The literature review indicates a correlation between ASI and HIC36, with evidence suggesting the relationship is likely exponential rather than linear. The report concludes that although ASI values above 2 may still meet MASH occupant impact and anthropomorphic test device (ATD) criteria, deployment decisions should consider the associated risk – particularly when installing bridge-parapet-like barriers on roadside.