On ALC we'd encounter lots of suportive drivers (and only 1 or 2 idiots) and you can tell the difference. Encouraging honking is a light tap on the horn, like "beep beep beep beep beep beep"

If they are yelling riders can't hear what they say but the "cheerful style beep" we know.

That's as opposed to laying on the horn right behind or anywhere near a rider. We all know and hate that. Even in a large group, century or charity ride you can't assume that sudden noise is not going to freak someone out.

Decorating your car with "Go MS 150 riders" or posting a sign is alright too.