Centerline miles are the actual length of roadway in one direction of travel. Opposing travel lanes on some state highways are separated by large medians, this can result in the total length of highway differing for each direction.
Centerline Miles in Washington in 2006
Source: WSDOT Transportation Data Office
The centerline mileage for highways may change as routes are modified through realignments. This can result in a lengthening or shortening of a route's total length.
For information about how roadways are used, click here: Vehicle Miles Traveled