Washington State Route 599 is a 1.75-mile freeway entirely contained within the City of Tukwila located in King County. WA 599 is a north/south route connecting I-5 to WA 99 and is former alignment of US Route 99.
This blog is part of the larger Gribblenation US Route 99 Page. For more information pertaining to the other various segments of US Route 99 and it's three-digit child routes check out the link the below.
Washington State Route 599
There isn't much to WA 599 as a route but there are some neat signs like this WA 599 "Begin."
The Light Rail from SeaTac International Airport crosses above WA 599 near Marginal Way.
There are only two exits on WA 599; the first is at Interurban Avenue while the second is at Tukwila International Boulevard. Tukwila International Boulevard is another former alignment of US 99 which predated the freeway WA 599 runs on now.
North of Tukwila International Boulevard the route becomes WA 99 and WA 599 ends. There is a much better sign assembly explaining the change south of this one, but I wasn't expecting it and missed the photo.
Interestingly there is a gap in WA 99 from WA 518 north to the terminus of WA 599 which is spanned but Tukwila International Boulevard. The routing of what is now WA 599 was constructed in the late 1950s and was part of Primary State Highway 1 WM (West Marginal). When the route had been completed US 99 switched alignments apparently onto a multiplex of I-5 and may have been once been signed as US 99T. The route has been designated at WA 599 since 1971. This older map of the state highway system from 1956 shows how US 99 used to flow through from Tacoma through Seattle before the construction of I-5.
1956 Washington State Highway Map
Edit 5/13/18: I was shown a 1970 map scan of downtown Seattle by Flickr user Arthur Allen showing the routing of WA 599 shown as WA 99T. The map scan can be viewed here at the following link:
Arthur Allen Flickr Page showing a 1970 Map Scan of downtown Seattle