Not strictly true, a while back someone passed me very close and I caught up with him very shortly afterwards when he got stuck in traffic. I pulled alongside him and just said something to the effect of "you passed me very close there sir, a little space would be nice". He had a look about him that I wasn't sure if he was going to launch into a tirade, accept what I was trying to tell him or be fearful that an angry cyclist was about to smash his windows or something. Anyway having moved on ahead I figured it would be interesting to see how he behaved if he passed me again (and was ready to take the rest of his number plate if a police report proved necessary). Anyway that time he passed me giving so much space you could have driven another car through it and waved (with all five fingers) so I waved back and that was the end of it.

Sometimes people are asshats, sometimes they're willing to accept they did something silly if you don't get in their face about it.