Severity Indexes for Roadside Objects

An attempt was made to develop severity indexes (SIs) for various fixed objects that are struck when vehicles leave the roadway. The objectives of the study included both the development of new indexes with recent data from two states and the exploration of methodological issues related to statistical modeling and unreported crashes. Both severe injury and cost-based SIs were developed for a wide range of crash situations by using data from both North Carolina and Illinois with which injury could be more precisely linked to the specific object stuck. Although the final SIs developed were not categorized by exactly the same control variables for the two states, the values of the indexes were, in general, moderately consistent between states and with earlier Texas-based work. However, the three state's SIs were not consistent with the SIs found in the AASHTO Roadside Design Guide. In the limited sample of airbag-related impacts with guardrails, trees, and utility poles, it appears that the airbag significantly reduces the value of the SI. Additional future research on the effects on SIs of both airbag-related and unreported crashes is needed.