How Weather Events Affect Freeway Demand Patterns

The ability to estimate freeway traffic demand during inclement weather events is critically needed for demand and supply management, level-of-service assessment, capacity verification, and other freeway operational and planning applications. Reliable travel demand prediction is also a useful input for freeway travel time reliability analyses and advanced traveler information systems. This paper investigates the effect of inclement weather on freeway demand with the use of hourly traffic and weather data collected for 5 years (2011–2015) at 15 urban freeway locations in the city of Milwaukee, Wisconsin, which represents a North American region experiencing distinct seasons with significant snowfall and cold temperatures in the winter. About 489,300 hourly traffic demand observations were used in the analysis, and more than 52,800 of those were under inclement weather. Freeway demand reduction factors were introduced for diverse weather variables, precipitation intensities, visibility levels, temperatures, weekdays and weekends, and different times of day. Variability across seasons and sites was also examined. The paper further explores traffic demand in the hours immediately preceding or following inclement weather events. Finally, freeway weather-related demand and capacity adjustment factors are compared. Lessons learned from the study along with recommendations for future Highway Capacity Manual editions are summarized.