I'm a courier, and spend at least a few hours of every day of the work week driving.
I've been smoking my pipe while driving lately, and clenching feels pretty natural now.
@juni - my jaw muscles were aching a bit during the first few times puffing most of a bowl in my teeth. But that soreness starts going away fairly quickly. I've also found it's easier and more comfortable to clench with your molars, as opposed to any of your teeth towards the front.
It also makes me smoker faster. And yeah you can't help but inhale a little more smoke than freehand, but this factor is pretty much eliminated if you are in even a slight breeze.