EFFECTS OF SUMMER PRECIPITATION ON URBAN-TRANSPORTATION

Potential shifts in summer precipitation due to an enhanced greenhouse effect indicate the possibility of more rain days and heavier rains i n the Midwest, and this study assessed the effects of such changes on transportation in Chicago using a 3-year period of data. Traffic accid ents in the metropolitan area doubled on rainy days, with 30% more acc idents in more densely populated urban areas than in suburban-rural ar eas. During rain events accident severity (number of injuries) was 55% higher in suburban and rural areas where less dense but higher speed traffic flows exist than in the city, however. Rain days during dry mo nths produced more accidents and injuries than during normal or wet mo nths. Three times as many accidents occurred during heavy rain periods (> 12.8 mm) as during nonrain conditions. Rain had a negligible influ ence on weekday traffic volume on busy highways but there was a 9% dec rease in traffic volume on rainy weekends. A 3-5% decrease in ridershi p of public transportation occurred on rainy days, with most decreases during midday. Nationally, 27% of all fatality-producing aircraft acc idents occurred during rainy weather conditions, as did 57% of the 30- min flight delays at Chicago's O'Hare Airport. Results suggest that gi ven continued transportation use patterns extend into the future, a fu ture climate with more summer rain days, somewhat higher rain rates, a nd more storms would mean more total vehicular accidents, more total i njuries in vehicular accidents, decreased ridership on public transpor tation systems, and more aircraft accidents and delays. A drier climat e would likely experience fewer moderate to heavy rain events but resu lts show that rain events during drier conditions produced a greater f requency of accidents and injuries per event than during wetter condit ions.