For a while I did nebs while driving my one hour commute to and from work. It was a great time saver, but I stopped because I really think it's quite dangerous to do it while driving:
1) It means I only have one hand on the wheel.
2) I had to open the window to prevent the cloud of meds around me, which meant I had to turn up the radio even louder ... i.e. I really couldn't hear anything going on around me.
3) Drivers nearby were so busy staring at me, that they definitely weren't watching where they were going.
4) When my phone rang I had to push the button on the handsfree to accept the call, and turn off the nebulizer so I could be heard. Obviously I wasn't very focused on the road while doing this.
In short, while it's tempting to save two hours a day by driving and nebbing at the same time ... I decided it's not worth getting into an accident.
Just my humble opinion ...