I once read, and have since proved to my own satisfaction, that most drivers give you as much room between their vehicle and yours, as you take between the edge of the road and yourself. I don't know why that is; it would seem that if you're 6" from the fog line, there'd be more room for a passing cager to fit his car than if you're 3' from the line, but many vehicles will pass you with 6" to spare in the first instance, and 3' in the second.

Most of the roads where I ride have deteriorating pavement near the edge, so I go look for a smooth stretch, which is generally found between tire tracks.