I have a terrible habit of automatically switching to sign language when I think people can't hear me, which includes people driving off in cars. Sadly I suspect that signing sarcastically with one hand while riding a bike is an artform under-appreciated, and perhaps liable to the wrong kind of misinterpretation, by the target audience.
Though there was that time I found myself behind a taxi in stop-start traffic, when a child of maybe 6 or 7 years old appeared in the rear window and decided to lecture me on the importance of wearing a helmet, and that I should ride on the pavement for my own safety, in fluent BSL. Sadly I had too much steering and braking to do to have much of a debate.