Weather causes a variety of impacts on the transportation system. While severe winter storms, hurricanes, or flooding can result in major stoppages or evacuations of transportation systems and cost millions of dollars, day-to-day weather events such as rain, fog, snow, and freezing rain can have a serious impact on the mobility and safety of the transportation system users. These weather events can result in increased fuel consumption, delay, number of accidents, and significantly impact the performance of the transportation system. The overall goal of the research work undertaken in this study was to develop a better understanding of the impacts of weather on traffic flow. The research was intended to accomplish the following specific objectives: (1)Study the impact of precipitation on macroscopic traffic flow parameters over a full range of traffic states; 2) Study the impact of precipitation on macroscopic traffic flow parameters using consistent, continuous weather variables; 3) Study the impact of precipitation on macroscopic traffic flow parameters on a wide range of facilities; 4) Study regional differences in reaction to precipitation; and 5) Study macroscopic impacts of reduced visibility. The work documented in this report was conducted in two parts: 1) literature review and development of a data collection and analysis plan, and 2) analysis and interpretation of the results. The recommended plan combined the use of macroscopic traffic data archives with archived weather data in order to meet the research goals that include achieving better understanding of the impacts of weather on macroscopic traffic flow. The results of the research conducted for this study were helpful in identifying weather impacts of traffic flow in the three cities studied, Minneapolis-St. Paul, Baltimore and Seattle. No impacts were found on traffic stream jam density, but both rain and snow did impact traffic free-flow speed, speed-at-capacity and capacity and parameters varied with precipitation intensity. The results of these analyses are documented in the report. This report concludes with some recommendations of future research related to weather and traffic flow. Several ideas are presented including enhancing the macroscopic analysis used in this study. Additional work is proposed related to human factors and microscopic traffic modeling.