 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 annual average 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 between SR 181 and the SR 167 interchange , serving 38 @,@ 000 vehicles , while the least busiest section was its western terminus at SR 509 , serving 8 @,@ 900 vehicles . SR 516 between I @-@ 5 and SR 167 within western Kent is designated as part of the National Highway System , which includes roadways important to the national economy , defense , and mobility . 
