The effects of weather on male and female driver injury severities: A mixed logit analysis

The dependence on motor vehicle travel in the United States has led to an increase in the number of vehicle miles traveled each year and, therefore, an increase in the exposure to the risks associated with such travel. In order to significantly reduce the number of annual injuries and fatalities associated with motor vehicle crashes, there is a need for a more comprehensive understanding of the factors that are involved in these crashes. One approach to expanding the knowledge of crash safety is to focus on the role of weather in automobile crashes. Existing transportation safety research that has considered the effects of weather has been limited in scope: considering only one or two weather conditions or analyzing the effects on a macroscopic level. This research provides an interpretation of the factors that affect injury severity based upon the analysis of crashes that occurred on multiple weather-related roadway surface conditions. The effects of weather on the injury severity outcomes of drivers of passenger vehicles who were involved in single-vehicle crashes on dry, wet, and snow- or ice-covered roadway surfaces are identified in this study. The probability of injury severity was estimated using data collected from police crash reports on the state-maintained roads throughout Indiana during 2007 and 2008. Likelihood ratio tests determined that parameters were estimated differently by roadway surface condition, driver gender, and driver age. As a result, 12 separate severity models were estimated using the mixed logit model with random parameters. This research provides a unique understanding of the factors that affect driver injury severities on ideal and adverse weather-related roadway surface conditions that will be beneficial to future transportation safety programs and designs. The average predicted probabilities for each injury severity outcome were determined for each driver group and weather-related roadway surface condition. Overall, adverse weather conditions increased the severe and no injury/PDO outcome probabilities, compared to dry surface conditions. The severe injury probabilities more than doubled in adverse conditions for female drivers and male drivers 45 years old and older. This study also identifies the increased risk of severe injury that male drivers under 45 years old experienced on average in dry surface crashes, compared to adverse surface crashes. Furthermore, several findings of this study support safety awareness and enforcement efforts already in effect (such as the importance of seatbelt use regardless of the weather-related roadway surface condition), while others have indicated additional areas of safety concern for specific driver groups