If they are just honking there horn going real slow I don't think it will effect the deer too much. Deer are used to cars and their horns. Unless the deer are super close to the road I don't think it will do much to move them. If they got out of there car and walked around then that would be a problem. It does stink though and I would be pissed as well if this was happening to me. I agree with the video camera idea if you have the time but if there are houses and a lot of cars that constantly go by I wouldn't worry about it effecting the deer. I can bet you the person doing it thinks that they are ruining your hunt, little do they know!