just saw a chart in this blog post that reminded me of this here question
http://andrewsullivan.thedailybeast.com/2012/01/the-economics-of-snow.html

They could be smarter about it when I lived in Philly not only did they ajce a ton of plows but the garbage and recycling trucks had snowplows additions they would add so they got that city's roads cleared quick. The main plows would do the thoroughfares and the garbage got the side streets AND picked up the trash. There were also alot of non-city owned plows that could be rented for places that weren't in the city center. It was amazing that theyd get a foot of snow but it was all plowed out of the streets in a few hours ( but trying to park with a wall of three foot snow was a bitch. People would put folding chairs in their "spot" that they had dug up. ) another thing that was kind of mind blowing is the night before everyone would pull up their windshield wipers so they looked like antena sticking out of the windshield but the next morning they weren't iced to the window. Just scrape away without bumping into them and slam them back down and go. But if you've never seen it before it looks really funny and you think the city has done some sort of mass prank until you see how quickly those people get back to work and just go. It's amazing to me being back and seeing the city shut down vs cities where it's just another day in winter. I hate driving in snow here because no one on the roads knows what they're doing and it's way scarier watching out for them

I thought this was an interesting statistic:

"193 plows are on the road between Olympia and the Canadian border."

from: http://www.wsdot.wa.gov/construction/2011/today.htm

That doesn't seem like very many. The population in that area is what, 6-7mil?

Anyway, I don't have any links or resources off of the top of my head, but Wasatch Valley (where Salt Lake City is) is of comparable size to the SeaTac area (though much less hilly), and I bet you can find out how many plows they have without much trouble. Growing up there, I can say the response is night-and-day different. In SLC snow is no big deal, it just gets plowed (and shoveled) away. The only time you don't really want to drive is during the worst of the actual storm itself. We had snow days less than once a year and I'd guess at least 12-24 inches would have to fall in 24 hours (and stick) before anyone even questions shutting stuff down.