This morning on the way to work, I was riding a lightly used four lane divided road with no shoulder. I see a SUV coming up behind me in my mirror in my lane and notice they're not changing lanes, I cover the horn, and drift left so I can shoot right and hop the curb if nessary. The SUV moves left at the last moment for a very close pass. As they pass I honk the horn but it didn't seem to register except for an after the fact glance back, apparently they were totally engrossed with an electronic device.
Not being clairvoyant, I don't know if it was done with malice or through inattention, but if the latter then perhaps they may have gotten the point.

It didn't save the day or change someone's course of action, but at least I was able to communicate my presence, and no one was the worse for it.