And US Highway 15 runs through South and North Carolina; I travel it every weekday on my way to work, and grew up in a house right alongside this highway! Wilt might have crossed it, where it runs concordant with I-95, on his way home in
Good Wilt Hunting, but since it's a north-south running highway that starts in SC on its southernmost end(in the town of Walterborough, between Charleston and Orangeburg)and runs to upstate New York, it could not have been on the west coast. It's already been established by the show's creators that Foster's is located in the Pacific Northwest, so that is canon.
EDIT- Sparky now has the screengrabs for "Pranks For Nothing" up on her site, and that particular screengrab in question is right here:
http://www.fosters-home.com/screengrabs/6/603-0073.jpg
As all can plainly see, that sign most definitely does say "I-5 FW", with a hyphen between the "I"(which is NOT a "1") and the "5", and the symbol to the left designates a Federal highway, as opposed to a state, county, or municipal road. That absolutely puts the hotel, at least, right off I-5, though it is probably located to the south of where Foster's is located, especially given that the Canadian border is just a 45-minute train ride to the NORTH of Foster's-definitely Pacific Northwest, though.
pitbulllady