I have my replays on auto-save, so if they do that they also have the bonus of a nice admin report
The game gives everyone all the information they need - yellow/blue flag messages, other car engines, sideways glaces. It's up to the user to actually use them. Granted, not everyone does. But if/when they do, there hopefully won't be as many people with pointless binds (and I include the horn in that category).
What I do if I'm not sure someone knows I'm there is bump my revs. I have a clutch bind even though I use auto-clutch 99% of the time, but if someone is driving like I'm invisible I'll engage it and hit the gas a couple of times. Don't know if it makes any difference, but it's something and it's less annoying than the horn