I-5 north from Seattle to Everett was opened in 1965. Most of the demolition downtown was done in 1962; I'm guessing the Northgate section was around the same time. I have a 1960 city directory that shows lots of buildings already vacant, about to come down; I haven't looked up by Northgate.

My guess would be topography, too; avoiding the hillside and swamp there. It's possible to build a freeway in conditions like that but the cost per mile is a lot higher. Land values may have had some impact, but remember, even as late as 1960 there wasn't much up there; Northgate itself was kind of in the middle of nowhere.

Damn fine answer, Sacre. And great info/dates, Fnarf!
I disagree that not much was up there, in one regard: there were a number of big craftsman Maple Leaf houses there from 1907 and later - right on 6th/5th and in the path, had I-5 gone straight North.

You're right; Maple Leaf was an established neighborhood. Maybe that's why; they could buy the undeveloped land to the west much cheaper than that already filled with quality housing and thus relatively well-off people. Freeways traditionally wipe out poor neighborhoods, not rich ones.

Note also that I-5 carved out the hill there at Banner/5th: the folks that lived there in the late 50's may have had deep clout. The houses in that area are older than the freeway by as much as 5 or 6 decades - The house on 79th/5th is circa 1907. The route 5th takes as an overpass used to be the ground-level.

Note also that north of the ship canal, I-5 was running nicely along the block that used to be home to 6th Ave before the freeway was put in. BUT it swerves East before it gets to Greenlake/Ravenna - why? John Marshall Junior High School, that's why - built 1927. They snuck the freeway between that and Calvary Church and the Apostolic Church on 75th.

I suspect the freeway swerves west again to avoid the power station at 75th and Fairview Christian Church (78th), as well as St. Catherine Of Sienna, further north on 8th (north of 85th).

Also; curves are/were included sometimes in interstate design to help prevent drivers from falling asleep.

Also, isn't that area around the east of Northgate kinda swampy? That might have been an obstacle at the time I5 was built.

Yeah, I thought about the hill, but I also thought that in the grand scheme of things, carving out a hill is not a big deal. When you think about the number of bottleneck traffic jams those curves create, several times a day, every day, forever, it seems like it would have been worth it to just have I-5 slope up or to carve out the hill. I'm no civil engineer so I don't know about the practicalities.

well, northgate itself was swampy - aka it used to thornton creek: which still runs under much asphalt (though the new development/cinema Thornton Place helped daylight some of it - and thus the name)