Leave an anonymous note at her residence. (on her car or in her mailbox) This can be done without letting on who the note was from.

Inform her that while motoring you saw what she did to that cyclist and did not approve. Adding a vague insinuation of possible retaliation can be helpful. Since she now knows that someone watching her knows where she lives, this will most certainly scare her straight better than anything else. No one would be comfortable in the letter receivers position.

When you see her pay her no special attention of any kind. Do not wave, do not go out of your way to ride by her house, do not approach her. (you didn't write the letter) If she wants to go to the police with the note (unlikely, as it would also incriminate her) it can not be pinned on you.

Edit: Great avatar Falcon!