I don't think you really make the point you're trying to, given that half the time you're waiting at traffic lights, so the rest of the time you have to be doin

I don't think you really make the point you're trying to, given that half the time you're waiting at traffic lights, so the rest of the time you have to be doing considerably more than 20mph to achieve that as an average. Hence waiting behind a cyclist at 15mph is slowing you down quite a bit.