Random parameter analysis of IIHS vehicle death rate factors and their contributions to fixed object and non-domestic collision severity

We study the severity of driver crashes involving fixed object and non-domestic collisions on the SR5, SR82, SR90, SR182, SR205, SR405 and SR705 in the state of Washington using a data set of 10871 for the years 2018 until 2019. We use a multinomial logit model to identify statistically relevant factors explaining the severity of the most severe injury type, which is classified into the four classes, which are non-apparent injury, possible injury, suspected minor injury and suspected serious injury plus fatality, respectively. Furthermore, to account for unobserved heterogeneity we use a mixed logit model with heterogeneity in variance. We study the effect of a number of factors including time period, sobriety type, vehicle count, work-zone information, first collision type, junction relationship, weather conditions, pavement surface conditions, ambient light conditions, first impact location, vehicle movement information, vehicle style, vehicle size, first vehicle action, vehicle defects conditions, vehicle 1 demographics, driver 1 contributing causes, site-type indicators, impairment & fault dummies and count, encroachment indicators, driver age level indicators, wildlife indicator, posted speed limits, presence of traffic control systems, age and gender of the driver and county locations of the crash. The objective of this study was to determine the contributing factors to vehicle driver crash severity involving fixed objects collisions. The results from this study need to be evaluated with caution due to the lack of data about specified driver behaviors and driver skills at the moment of crash related cases available in the WSDOT crash database. Implications for identifying and improving the reporting of unobserved driver behaviors, driver skills and other related factors are therefore discussed.