This post originally appeared on Orphan Road.

When I first heard mention of a $1B line-item in the RTID to connect SR 509 with I-5, I assumed they were talking about eliminating the stoplight where 509 meets 518 and making it an “all freeway” exchange. It struck me as an odd thing to spend a billion bucks to get rid of a single traffic light.

But I was wrong! The proposed connection, which is nearing approval would happen south of Sea-Tac. The P-I article, though, still doesn’t answer the question of what problem the expansion is designed to solve.

WSDOT’s website, though, provides an answer:

Extending SR 509 will ease congestion on I-5, improve service between industrial districts by allowing up to 9,000 trucks per day to bypass I-5, SR 99 and local streets, and provide for southern access to Sea-Tac International Airport.

It also seems like calling it a “509 expansion” is a bit misleading: in addition to the 3 miles of new 509 freeway, the project will also add a lane to I-5 for the 6 miles approaching the 509 interchange. I’m sure the 509 piece is more expensive (because it’s brand-new freeway), but still, a good chunk of this project is widening I-5.