Depends on the situation but sometimes I find that a car trying to squeeze past can be deterred by me holding out my hand as though it might be a signal. When they see you only have the one hand on the bars and appear (are) less stable, they begin to register the possibility they are a danger to you and might slow down. Of course if they have the much discussed "sense of entitlement" it probably won't work. That's why I said in my last post that we need more anarchy.