SR 509 - I-905

WSDOT has long-range plans for extending the freeway portion of SR 509 to connect to I-5 south of Sea-Tac Airport. Currently it heads south on 1st Avenue from SeaTac to connect to SR 516 in Des Moines.

The north end of SR 509 is at the 1st Avenue Bridge over the Duwamish River, where SR 99 comes in from the side and takes over 509's freeway. After a mile as a surface street on East Marginal Way, SR 99 turns back into a freeway on the Viaduct and Battery Street Tunnel.

Since the Viaduct and tunnel are going to be replaced in the next six years, I would suggest that the East Marginal Way section be turned into a freeway and frontage road combination, so that the freeway is continuous from SeaTac to the north end of the new downtown tunnel.

Once WSDOT connects that freeway to I-5 south of the airport, it would be an alternate high-speed route all the way to downtown Seattle, and thus worthy of an Interstate number. I propose 905 to keep the numbers familiar. Plus it fits the Interstate numbering requirments as a spur (non-loop) of 5.

SR 99's north segment would therefore start at the north end of I-905, north of downtown (or possibly at Green Lake?), instead of in Tukwila. 99's portion of the SR 99/599 freeway could easily be transfered to SR 599, and perhaps that junction at the south end of the 1st Avenue Bridge could be reconstructed to be a full-speed all-directions interchange (instead of its current huge backtrack loop alignment for southbound 99).

SR 99 and SR 509 both have southern segments with other oddities, but I'll discuss those later.