Transportation agencies are seeking to integrate weather data into traffic operations in order to improve system efficiency. In order to do so, it is essential that transportation professionals have a solid understanding of the impact of various weather conditions on traffic flow. The purpose of this research effort was to investigate the impact of rainfall, at varying levels of intensity, on freeway capacity and operating speeds. The findings of this study, derived from traffic and weather data collected in the Hampton Roads region of Virginia, add to the guidance currently available to professionals in the Highway Capacity Manual . The findings are summarized below. • Light rain (intensity of 0.01 - 0.25 inches/hour) decreases freeway capacity by 4-10% • Heavy rain (intensity of 0.25 inches/hour or greater) decreases freeway capacity by 25- 30%. • The presence of rain, regardless of intensity, results in approximately a 5.0-6.5% average decrease in operating speeds. These findings indicate that the impact of rain is more significant than currently reported in the Highway Capacity Manual , and therefore indicate the need to carefully examine freeway operations strategies during rainfall events. Smith, Byrne, Copperman, Hennessy, and Goodall 1 INTRODUCTION It is widely accepted that weather plays a significant role in the performance of the surface transportation system. Certainly, extreme weather events, such as snow or thick fog, can bring traffic to a standstill. However, beyond these extreme conditions, more common weather events, such as rainfall, have also been shown to impact traffic conditions. As the transportation profession continues to seek to operate transportation facilities at the highest possible level of efficiency, it is necessary to take into account the impact of weather. In a recent article entitled “Weather: A Research Agenda for Surface Transportation Operations,” Nelson and Persaud argue that “people who manage highway operations must seek new techniques and intelligent transportation systems that complement the amazing systems of weather-information collection, analysis, and forecasting” (1). There are a number of examples of system configurations that can meet this challenge. For example, a freeway ramp management and control system could be designed to automatically modify metering rates based on changes in effective mainline capacities due to weather conditions. However, in order to effectively integrate traffic operations with weather information, one must have a solid understanding of the impact of weather conditions on key traffic parameters such as capacity and operating speed. The purpose of this research effort was to investigate the impact of rainfall, categorized by intensity, on freeway traffic flow. In particular, this research expands on the limited guidance in this area currently presented to freeway operations personnel in the Highway Capacity Manual . This guidance, as well as previous research in this area, is summarized in the next section. LITERATURE REVIEW In Chapter 22, “Freeway Facilities”, the Highway Capacity Manual (2) addresses the impact of rain on freeway capacity and operating speeds. The manual first