That just leaves High Speed Ring, Autumn Ring and Grand Valley without any clues as to where they are situated. Although I can remember way back someone on here speculated that HSR could be set in the Netherlands.
Hmmm, being from the Netherlands, I can't imagine any place (and I've seen a lot of my country) in the Netherlands where that could be based upon, I always got a Germany vibe when driving the HSR, but that might just be because of its name.
Also someone did a study on the feasibility of Apricot Hill and found its topography to mach up really closely with south eastern Ohio. I don't think that's where they'd put it given the choice but it's neat none the less.