rsbob 

I, for one, do not mind a quick toot of the horn as a warning I am about to be passed. Now laying on the horn, is rude and aggressive, but a quick toot is just fine with me. I sure don’t do it, because I don’t want to risk startling a rider, but certainly don’t take offense if someone else does it. I usually give them a quick wave out of appreciation, just like those who move into the other lane to give me room while passing. To each their own.