Ok, I´ve got back from a ride, so here goes:

Dear Driver,

I am sure that you found it very amusing that I fell off my bike in front of you. I understand that it was pure incompetance on my part that left me getting closer to the road than I had intended. I don´t suppose that it occurred to you to ask if I was ok, so I´ll let you know that I am.

Thanks for not asking, Bron