When the current state highway numbering system was first posted in 1964, SR 509 began at Jct SR 16 in Downtown Tacoma and headed NE across the port area via E 11th St to Marine View Drive, then took Marine View Drive and Dash Point Rd out past Browns Point and Dash Point to Pacific Hwy S north of Federal Way. This portion of Pacific Hwy S had been part of US 99 before 1962 and had become Business Loop 5 by 1964. In 1969, it was redesignated as SR 99. SR 509 joined BL 5 or SR 99 and headed north for about a mile. At 16th Ave S, SR 509 left the Pacific Hwy S and headed to Downtown Des Moines via Saltwater State Park. From Des Moines, SR 509 continued north along 1st Ave S and Meyers Way via Burien to the 1st Ave S Bridge, where SR 509 ended at Jct US (later SR) 99.
In the late 1960s, SR 509 was moved to a freeway from Burien to the 1st Ave S Bridge. This SR 509 freeway was extended a couple of miles south in the later 1970s. In 1972, SR 16 was rerouted from I-5 towards the Tacoma Narrows Bridge via the Nalley Valley Viaduct. SR 509 was extended south from Downtown Tacoma to I-5 via Pacific Ave, former SR 16.
In the 1990s, significant changes were made to SR 509 in Tacoma. SR 509 was shifted to a new route around rather than through the Port of Tacoma area. This new route was a combination of existing city streets plus new construction. The new construction included a cable-stayed bridge across the Thea Foss (formerly City) Waterway. Old SR 509 was dropped from the state highway system and the Blair Waterway Bridge on E 11th St was removed.
Effective April 1, 1992, the route of SR 509 from Federal Way to Des Moines was changed. Instead of leaving Pacific Hwy S at 16th Ave S and serving Saltwater State Park, SR 509 now continues along SR 99/Pacific Hwy S to Jct SR 516 at Midway. At Midway, SR 509 now leaves SR 99 and joins SR 516 heading west to Downtown Des Moines. In Downtown Des Moines, SR 516 ends and SR 509 rejoins its traditional route. The SR 509 signs along its former route have been removed but no SR 509 signs along the new route have been posted as of October 2001.
Future plans call for extending the SR 509 freeway near Burien around the south end of Sea-Tac Airport to the east to connect with I-5 between Exits 149 and 151. Today, SR 509 begins at Jct I-705 in Tacoma and ends at Jct SR 99 at the south end of the 1st Ave S Bridge in Seattle. SR 509 is 35.17 miles long.
Before 1964, SR 509 from Tacoma to Des Moines had been SSH 1V and from Des Moines to Seattle, part of SSH 1K.
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