I don't drive personally, but when I'm in a car - I tend to sign to myself, even if I'm in a car with people who don't know sign language.

I get lots of weird looks.
I've also started finger spelling signs more than often, but not just with one hand, but with both hands at the same time.

Which helps me get better at signing left handed (as I'm a right dominant signer, but I am ambidextrous in signing - just a little weak with signing and finger spelling left handed, but I'm getting better. :])