40 and below, I don't worry about it if there is another lane for cars to go around and traffic is not really dense. People change lanes to go around old ladies and semi trucks all day long, a bike shouldnt be any different. I generally avoid roads with only one lane going my direction and no shoulder unless there's a lot of lights to keep traffic slow.

Really, to a car, they don't see the difference between you going 12 and 18, you're still slow and annoying.