On another thread yesterday, someone was adding up the numbers to arrive at a safe lane width mathematically, which is what made me realize I wouldn't know anyway. My own math would go that a bike probably takes up 3' of space on the side of the road (tire tracking 2' from the usable edge, and the left side of the bike and rider is another 1' beyond that), then 3' for safe passing = 6' already gone. Someone was using 7' for a standard motor vehicle width; I don't know much about that either. But assuming that, that gives 13' as the absolute minimum shareable width.
Bek, 18' means they could give you up to 8' of passing space! However, another factor that might play into that is that I think there is a subconscious tendency in all drivers, motor and otherwise, to want to occupy the exact middle of the available space they have, rather than to one side or another of it. It makes sense that you would want to give yourself an equal buffer on both sides. This means that a motorist might not even take all the room that they have available on their left, so just to say they have N' of space doesn't mean that they'll necessarily take it. They may just take N/2' of it.