When I encounter a D-head like the one you encountered, either while riding or driving, is to pretend I have a rocket launcher, and just imagine him (or sometimes her) being blown to bits. I know it shouldn't but it helps me forget about it and go on with my day. I would never engage with someone like that. People that do stuff like that are not right in the head in the first place and you don't know what they might be capable of. Besides that, its highly unlikely that anything you say or do would get through to them anyway and that will just anger/frustrate you more.