I've mentioned this in a couple threads (wish list, etc) - but, both the phone and watch need to be more car aware. The phone should know, based on its bluetooth pairing that it's in the car and give that handshake to the watch.
Then, the watch would understand not to come on as you're using your hands for driving. (needs to also understand the subtlety of a lift raise vs a wheel motion).
Lots of other good things that would come with that - Siri audio when in the car, have your iPhone do most of the work (because it's likely plugged in).
Ideally, Apple should make all of this highly customizable by the user. (if I find the watch microphone to be more responsive than my built in hands free, please use the watch mic - for example).
Just one of the many areas for improvement I'm sure we'll see over the coming months / years with these. 🙂