It has been widely accepted that weather has a significant impact on road safety. The large body of literature points out that weather is an environmental factor that affects both frequency and severity of crashes. Research has shown that especially adverse weather conditions are associated with increase in the numbers of crashes and rates. Furthermore, the prevailing weather conditions may influence the severity outcome of a crash. However, weather may be a factor that modifies crash conditions and not the major cause of a crash. Thus, any interactions between the prevailing weather conditions and other crash specific variables especially on crash severity should be taken into account.

In view of the above, the purpose of this thesis is to investigate the interaction effects of the prevailing weather conditions in combination with other crash characteristics on crash severity. To do so, a study on two different corridors in Iowa was conducted. Specifically, road segments from two different facilities, an Interstate route (I-80) and a US route (US-34), were selected and the corresponding crash severity was examined by estimating discrete outcome models.

The estimation results show that adverse weather conditions when interacting with other crash attributes influence crash severity. Among the weather conditions, temperature below freezing, precipitation (type and amount) and wind (speed and direction) were found to contribute to the severity outcome of crash. However, the combination of the prevailing weather conditions and route classification may have diverse effects on crash severity. For instance, adverse weather was found to reduce the probability of very severe crashes on the interstate study corridor, while the opposite effect was observed on the US route corridor.

The results of this thesis could provide transportation agencies with useful insights about the maintenance and operation activities that should be undertaken on different roadway facilities, especially under adverse weather conditions. Finally, the findings of this study can have potential implications in driver education as well as informing road users about the various effects of weather on safety with an emphasis on safe driving under inclement weather.