Wash. Rev. Code § 47.17.160
A state highway to be known as state route number 99 is established as follows:
Beginning at a junction with state route number 18 in the vicinity of Federal Way, thence northerly by way of Midway, to a junction with state route 518 in Tukwila; also
Beginning at a junction with state route number 599 in the vicinity of Tukwila, thence northerly by way of Seattle, Edmonds, and Lynnwood to a junction with state route number 5 in Everett: PROVIDED, That until state route number 509 is constructed and opened to traffic on an anticipated ultimate alignment from a junction with state route number 705 in Tacoma via the Port of Tacoma industrial area to a junction with state route number 18 in the vicinity of Federal Way that portion of state route number 99 between state route number 5 at Fife and state route number 18 in the vicinity of Federal Way shall remain on the state highway system.
RCW 47.17.160