It's Canada. I found the exact location. To avoid doxing the poster, I'm going to not give the specifics, but I've found the location pictured. It's in Canada.

Those are not Californian plates, which say California in large red text at the top. The text at the top is in blue, like Ontario, Quebec, or British Columbia. US interstate highways aren't the only roads that have red and blue shields, some Canadian highways have similarly colored shields. US interstates do not place mile markers on the left side of the road, they are always placed on the right side.