A procedure to account for the impact of rain and congested conditions on the average speed estimates is provided in this study. Although the Highway Capacity Manual (HCM) provides some discussion on the impact of adverse weather on speed-flow relationships, these impacts are not quantified. Using data collected under rain and congested conditions, a procedure for estimating the average speed under these conditions is provided, which is an improvement over the existing HCM (2000) procedures. Using the speed-flow relationships provided in the HCM (2000) for basic freeway segments as a starting point, new numerical relationships suitable for New Jersey roadways are derived. The new speed-flow relationships can be used to estimate operating speed and level of service (LOS) for New Jersey roadways under rain and congested conditions. The findings are as follows:

• The speed-flow model developed in the research can be used to describe conditions under clear weather, rain, and congested conditions. The model reflects the fact that as flow increases, speed decreases under clear weather and rain conditions. Under congested conditions speed and flow operate on the lower or congested portion of the speed-flow model. In this case, as more vehicles are added, the discharge flow decreases and the speed also decreases. The speed under rain and congested conditions is higher than the speed under congested conditions.
• Under rain conditions the average speed decreases by about 0.05 mph when the precipitation level is 0.01 inches/hr.
• Both the speed-flow model developed in this research and the HCM (2000) show that the average speed under rain conditions seems to decrease slowly when the flow rate is less than 2000 vphpl. However, the rain adjustment factors, developed using individual roadways reflect the fact that the average speed under rain conditions seems to decrease significantly at low to medium flows and decreases slowly at medium to high flows.