 This is expressed in terms of average annual daily traffic (AADT), which is a measure of traffic volume for any average day of the year.In 2011, WSDOT calculated that the busiest section of SR 501 overall was west of I-5 at C Street, serving 13,000 vehicles, while the least busiest section of SR 501 overall was south of Shillapoo Wildlife Recreation Area, serving 650 vehicles.Between I-5 and the Port of Vancouver, SR 501 is designated as part of WSDOT's Highway of Statewide Significance, which include interstate highways and other principal arterials that are needed to connect major communities in the state of Washington.