It's not always futile...

I had a van buzz me while the passenger screamed out the window (six inches or so from my ear) to 'Get the f*** off the road!' ...

I chased them two miles, with them circling around and heading back the way they came from (although on a different road), as the van seemed unwilling to let me find out where they lived / worked / wherever they were going. Lost them at a red light (they ran it!) and they got away.

Two weeks later, the same van comes up behind me, slows down, then passes me, with about 10 feet between me and him. I just smiled and waved.

I guess they didn't want the insane cyclist chasing them down again.

In this case I didn't even get a chance to confront the driver, they were already past me before I recovered my wits enough that I could have responded, but it obviously had an impact on the driver.