 Every year, WSDOT conducts a series of surveys on its highways in the state to measure traffic volume.This is expressed in terms of 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 northern section was the I-5 interchange, serving 11,000 vehicles, while the least busiest section was the intersection with Main Street in Downtown Ridgefield, serving 3,200 vehicles.