Washington State's highways are designated by the Washington Legislature and codified in chapter 47.17 RCW.  The state highway routes are defined generally by termini and points along the route.  State Route 501 (SR 501) is a 14-mile route in Clark County located in Southwest Washington.  The SR 501 is split into two sections:  (1) a 3-mile northern section that runs east-west through the city of Ridgefield and serves as the main street in Ridgefield; and (2) an 11-mile long southern section that begins at an interchange at Interstate-5 (I-5) near downtown Vancouver and follows the Columbia and Lake rivers north, ending south of Post Office Lake in the Ridgefield National Wildlife Refuge.  The southern section is designated as the "Erwin O. Rieger Memorial Highway."

The 3-mile split northern section of SR 501 within the city of Ridgefield that begins at Pioneer Street, at an intersection with Main Avenue at the Ridgefield City Hall, and continues east through Downtown Ridgefield and a couple of roundabouts, before ending at a diamond interchange with I-5 is removed from the state highway system.  The 11-mile split southern section known as the "Erwin O. Rieger Memorial Highway" remains SR 501 in the state highway system.