A highway renumbering in 1964 introduced the sign routes that would be co-signed with the existing system until 1970, one of which would replace SSH 1A, SR 9.

The projects ranged from expanding the current weigh station at the Soper Hill Road intersection in 2005 to realigning the highway between Nooksack and Sumas in 2006 to eliminate 90-degree turns.

Some projects include widenings, adding a roundabout at SR 531, realignments south of Snohomish and the addition of web cameras.

SR 9 travels north along the west side of the Brightwater sewage treatment plant and a community park in Maltby.

[3] The four-lane highway continues north and intersects SR 524 at Turners Corner before turning northeast towards Clearview and Cathcart.

SR 9 travels along the eastern ridge that overlooks the Snohomish River valley and has several sections with median barriers to form a divided highway.

North of the Snohomish River Bridge, the highway encounters a diamond interchange with 2nd Street and Riverview Road and turns northeast to intersect Bickford Avenue, which once was U.S. Route 2 (US 2).

After Lake Cassidy, the road intersects SR 528 and continues into North Marysville, where the roadway passes over the Snohomish County Centennial Trail.

[11] The road passes through Big Lake and its community of the same name before intersecting SR 538 at a roundabout east of Mount Vernon.

At the end of the concurrency, the road turns north as Township Street, paralleling another BNSF rail line,[12] at Cascade Middle School.

Continuing north out of the city and into rural areas, SR 9 crosses the Samish River and exits Skagit County.

[8] The current route of SR 9 began as a road extending from Grace (today Woodinville) north to Snohomish, first appearing in an 1895 map.

[19] Much of the highway was not complete at the time, leading to calls from the Associated Clubs of the North End to accelerate construction to provide an alternate connection to Canada.

[21][22] The corridor was also incorporated into the East Pacific Highway, a designation created in 1951 as an alternative route to US 99 between Tenino and the Canadian border at Sumas.

[36] The Lake Stevens weigh station, located on the west side of the highway at the Sopher Hill Road intersection was expanded to serve two trucks at once in late 2005.

Since late 2005, WSDOT has been improving the corridor with six projects located between SR 522 and the Skagit County line that are scheduled to be completed by 2013.

[48][49] South of Snohomish, WSDOT is improving the highway in multiple ways including new intersections and alignments, new web cameras and new turn lanes.