During the 1964 highway renumbering, the branch became a section of SR 303, which ran from Bremerton to Bangor and had a spur route serving Keyport.

[3] The highway travels east through wooded land, intersecting several minor roads until the roadway turns northeast and crosses over Liberty Bay into the unincorporated community of Keyport.

Within Keyport, SR 308 passes the Naval Undersea Museum and serves as the main thoroughfare through the community before it ends at the main entrance to the Naval Undersea Warfare Center, continuing east as Strom Avenue into the base.

In 2011, WSDOT calculated that the busiest section of the highway was between SR 3 and Liberty Bay, serving 11,000 vehicles, while the least busy section was the bridge over Liberty Bay, serving 6,300 vehicles.

[6][7] The branch traveled 2.34 miles (3.77 km) east from PSH 21 near Naval Submarine Base Bangor to the Naval Undersea Warfare Center in Keyport, intersecting Secondary State Highway 21B (SSH 21B) before crossing Liberty Bay.