Hmm... do you remember one of the Death Races where you start on a piece of concrete underneath a flyover? The race itself was all downhill, with no traffic whatsoever. If you can remember that, then follow the course but slowly. Down that empty stretch of road should be the entrance somewhere on the left.
If you look on the map, inbetween the top two areas should be a stretch of road that looks like a handle, on the far left and on it's own. That's the road.