I'm not sure why anyone would do a pre-scheduled phone interview while they were driving, but since I've recently discovered that some people do in fact engage in this behavior, please consider this a formal suggestion not to.
It baffles me that this happens, because:
1. First of all, you can't focus fully. Why, why, why wouldn't you want to be able to focus fully on presenting the strongest case for your candidacy possible?
2. Second, it denies you one of the greatest benefits of a phone interview: the ability to have notes in front of you.
3. Third, has it occurred to you that you might have to honk at someone or slam on your brakes or that you might get rear-ended (or worse)? Obviously I hope you don't get rear-ended for entirely different reasons as well, but as far as the interview goes, why are you that cavalier about possible disruptions?
4. Fourth, you're sort of signaling that you're not prioritizing the interview enough to even pull over to the side of the road, and that's not a great impression.
5. And fifth, there's a not-insignificant number of people who are staunchly, passionately against talking on a cell phone while driving. I once had an industry VIP refuse to speak to me when he realized I was answering my phone from my car. (To say nothing of the in-your-face, screaming meltdown I watched one of my neighbors unleash on someone last week for driving while talking on his phone.) If your interviewer feels even a little like that, do you really want to jeopardize your interview performance over cell phone politics?
Frankly, I wish everyone would use a land line for phone interviews, although I realize my dream is becoming increasingly out of reach. But at least pull over to the side of the road.