Weather plays a significant role in the operation and safety of the nation's roads. Each year billions of dollars are lost to accidents and delays due to weather-related conditions. It is estimated that weather-related delays for trucking companies are upwards of $3.5 billion annually. Additionally, approximately 17% of all fatal accidents and 7,130 persons are killed in weather-related accidents each year. It is important to understand weather-related accidents as it can result in lives saved and substantial monetary savings. Weather-related accidents are defined as those that occur in the presence of adverse-road or adverse-weather conditions. Adverse-road conditions are wet, snow covered, or icy pavement and adverse-weather conditions are where an atmospheric condition such as rain, snow, sleet, fog, or crosswinds is present. Many previous weather-related studies investigated accidents at the point level or as a metropolitan area, and do not differentiate between vehicle types. Studies that examined broader spatial extents, such as individual states or larger regions, often do not differentiate between vehicle types either. An understanding of the spatial and temporal variations in weather-related accidents can highlight ideal times for driver education and safety programs in regards to adverse-weather.

This study examines fatal crashes; 20,222 large commercial truck (LCT) and 87,217 passenger vehicle, in order to gain a better understanding of the temporal and spatial distribution of weather-related fatal accidents on U.S. Highways and the role of weather conditions from 2000-2009. Five spatial extents, national plus four regions, were used. The four regions include the Northeast, Midwest, South and West.

Nationally and in the three regions with regular snow and ice, a strong seasonal cycle of adverse-weather and adverse-road related accidents exists. Of the four regions, the Midwest region has the greatest average intra-annual variability for both LCTs and passenger vehicles. For LCTs, the average monthly peak occurs in January with 38.29% of accidents occurring with adverse-weather, and a minimum in June of 8.27%. For passenger vehicles, accidents are less affected by adverse-weather with an average intra-annual peak of 30.28% in January and a minimum of 6.32% in June. The South region has the least average intra-annual variability of accidents in adverse-weather, having a peak of 19.56% in January and a minimum of 9.35% in August for LCTs and 16.00% in February and 8.30% in May for passenger vehicles. Adverse-road related accidents are greater in all regions than adverse-weather due to the fact that accidents can occur on wet or snow/ice covered roads in the absence of adverse-weather.

Winter months for all regions contain the greatest inter-annual variability with weather-related ranges as large as 48.19% for LCTs and 42.56% for passenger vehicles, both in the Midwest region. The South region has the least winter inter-annual variability with December and January having ranges under 20% for LCTs and 15% for passenger vehicles. The summer months for all regions have minimal inter-annual variability, often with ranges 20% or less for LCTs and 10% or less for passenger vehicles. The West region has the least summer inter-annual variability with ranges 15% or less for LCTs and 7.5% or less for passenger vehicles. The Northeast region has the largest range with LCTs as high as 31.25% and 21.15% for passenger vehicles.

In addition, this study examines links between anomalous temperature and precipitation on a regional scale. Relationships such as increased percentages of weather-related accidents during above average rainfall appear, however do not exclusively hold true. The same holds for below average rainfall as many months have reduced accident percentages, though not all. Discussion of data, difficulties in analysis, application of results, and possible future work conclude the analysis.