Every year, the Washington State Department of Transportation (WSDOT) conducts a series of surveys on its highways in the state to measure traffic volume. 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 the highway was the SR 500 concurrency in Orchards, serving 34,000 vehicles, while the least busiest section was at the western terminus of SR 503 Spur east of Yale, serving 720 vehicles. SR 503 between Orchards and Battle Ground is designated as part of the National Highway System, which includes roadways important to the national economy, defense, and mobility.