Two other roads—a Cascade crossing at present State Route 410 and a branch of the first road to Wenatchee—were added in 1897.

The Washington Highway Department was established in 1905, and a set of twelve State Roads, numbered from 1 to 12, were assigned.

The first major reworking of the system was passed in 1937, including a complete renumbering.

By 1952, the present highway shield, in the shape of George Washington's head, had been adopted.

[2] The signs for the new highway numbers, which would be organized based on their general direction and—for secondary routes—a leading digit, first were posted in late 1963 and took effect in January 1964.

Routes and new Interstate Highways to reduce confusion and eliminate duplicate numbers.

It also served U.S. Route 99 Alternate (now SR 11) in Skagit and Whatcom counties.

Prior to the construction of the first floating bridge across Lake Washington in 1940, the route from Seattle to Issaquah followed what would later become SR 900.

Spurs extended along I-82/, SR 125 and SR 129 from Tri-Cities, Walla Walla and Clarkston to the Oregon State Line This route was also known as the "Inland Empire Highway" and crossed the first, and for a time, the only highway bridge over the middle Columbia River.

[8] PSH 5 had several branch routes that connected to Renton, Tacoma, Mount Rainier National Park, and Cayuse Pass.