If I have time, I use the horn, and if not, I scream. I had to overcome a psychological hurdle, though, for the screaming to become an instinctive reaction, as for myself and most people the honed response is not to cause trouble. This is similar to the case of people dropping off a ladder and instinctively protecting the can of paint and not the spine. Sometimes my riding partner strikes a critical tone with "You screamed, but nothing happened." They were the person who ended up in ER, illustrating how far people lean towards politeness. Incidentally I do not recall similar comments when I used the horn, presumably because the situation played itself over a longer time and one could more easily tell the details.