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 2012, WSDOT calculated that the busiest section of SR 18 was its interchange with SR 167 in Auburn, serving 97,000 vehicles, while the least busiest section of the highway was its eastern terminus at I-90, serving 19,000 vehicles. SR 18 is designated as part of the National Highway System for its whole length, classifying it as important to the national economy, defense, and mobility. WSDOT designates the entire route of SR 18 as a Highway of Statewide Significance, which includes highways that connect major communities in the state of Washington.