Honestly, I simply explained things. Using whatever terms and analogies seemed to meet her where she was at the time. She understood, for example, that a car is very big, very powerful, and carries a tremendous amount of energy as it moves... therefore, it's VERY IMPORTANT to operate the vehicle safely and to make sure that you don't hit anything. It took time (witness the side mirror incident) for her to understand that there are times when I simply can't be talking with her in the car while I'm driving... but honestly, I think that her awareness of the surroundings/driving conditions is ultimately going to make her a more aware driver in a few more years, too.
We use the same approach, and it's already paying dividends with DD8, when we take her to ride her bicycle in the nearby cul-de-sac at the very end of an otherwise busy street. She notes how drivers are extraordinarily lazy at turning on those turn signals, but she waits patiently each time, anyway.
Well, it's a combination of explaining everything, plus the natural consequences thing I mentioned earlier. The consequences were a necessary first step to get DD listening in the first place.
