 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 between SR 3 and Liberty Bay , serving 11 @,@ 000 vehicles , while the least busy section was the bridge over Liberty Bay , serving 6 @,@ 300 vehicles . 
