Seattle is known for tree-huggers, Earth-firsters, moss-between-the-toes nature lovers. So, here's a story about how Seattle got this way by leveling hills, filling valleys, and rerouting water - and even knocking itself down when it gets in its own way.
An Interstate runs through it. You know that. Yes, it looks like the highway came first and then the city built up, around, under, and over it; but, I-5 has only actually been there for about 60 years. Prior to that, Seattle was a seamless city from the waterfront of Elliott Bay, up and over Capitol Hill, and down to the waterfront of Lake Washington. Interstate 5 stitched Mexico to Canada with a thread of concrete 1,391 miles long; while also cutting Seattle in two. The city's been trying to stitch itself back together since.
Flip Route 66 and get Route 99. Similar system, different roads. Route 99 made it almost all the way from border to border; but the Interstates were meant to be broader, faster, and free from traffic lights, eventually. Route 99 had enough trouble making it through downtown. Look at Seattle's geography and it becomes obvious that there are no easy routes in, through, or around the city. Lakes and hills get in the way. Oh yes, and there were skyscrapers there, too.
Debate the pros and cons of highways, but regardless, Seattle intended to benefit from the highway. Rather than cut through downtown, or climb up our hills and run along our ridges, a middle course was selected. It wasn't an easy course, because it necessitated the construction of the Ship Canal Bridge, extensive elevated roadways for much of its length as it skirts Capitol Hill, and an eventual return to familiar concrete on dirt south of the city.
Running a highway through the middle of an active and growing city is more than just widening some streets. On and off ramps need room. Crews need room to work. And then there are the express lanes. A swath of buildings and neighborhoods, some pre-dating the Great Seattle Fire, were sacrificed. Easy walking routes were cut off requiring long detours, reasons to drive instead of walk, or creating incentives to not make the trip. People were not happy, but the concrete must be poured; especially considering its military inspiration and the Cold War climate.
Since then we've gotten used to it, and the effort to reconnect the city continues. The Washington State Convention Center ties the two via the bridge it was built upon. Freeway Park and Colonnade Park help as well.
In general though, Seattle has a concrete border that delineates downtown from up on the hill, that helps the majority of commuters get into and out of the city, and that puts the city in the West Coast's flow of international commerce. It was a grand plan, and one that was completed - not like the ramps to nowhere along SR520, but the fate of the Thomson Freeway is another story.