An Investigation into the Impact of Rainfall on Freeway 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. Findings were derived from traffic and weather data collected in the Hampton Roads region of Virginia. Light rain (0.01 to 0.25 inches per hour) decreases freeway capacity by 4 to 10 percent. Heavy rain (0.25 inches per hour or greater) decreases freeway capacity by 25 to 30 percent. The presence of rain, regardless of intensity, results in approximately a 3 to 5 percent average decrease in operating speed. The findings indicated that the impact of rain is more significant than currently reported in the Highway Capacity Manual.

Estimation of the Effect of Rain and Incidents on Freeway Capacity and Free-Flow Speed

This research aims to examine the effect of incidents with lane closures and adverse weather conditions (medium to heavy rain intensity) on capacity and free-flow speed (FFS) of freeway segments. Data were collected from multiple freeway segments located in the Kansas City, U.S., metro area from 2014 to 2018. The capacity and FFS were measured for two-lane, three-lane, and four-lane freeways under four conditions: ( 1 ) base conditions, ( 2 ) adverse weather only, ( 3 ) incidents only, and ( 4 ) adverse-weather-and-incidents. Capacity adjustment factors (CAF), and speed adjustment factors (SAF) were established to identify the remaining capacity or the FFS reduction during an incident or adverse weather conditions. The findings indicated that medium to heavy rain resulted in a 5% reduction in FFS at three-lane sites which is consistent with the adjustment factors shown in the Highway Capacity Manual 6th edition (HCM6); however, rain was not found to have a significant impact on freeway capacity. It was also found that incidents leading to one-lane closures reduced capacities by 30%, 17%, and 17% at two-lane, three-lane, and four-lane sites, respectively. Incidents were also found to reduce FFS by approximately 5%–10%, possibly because of “rubbernecking.” Adjustment factors that capture the combined effect of incidents and rain on FFS and capacity are also presented.