I'm really glad you asked this because I've been wondering about something related to this. Honestly, I've never had much luck with listening to the scanner while driving. I have a Radio Shack Pro-94 handheld. I'll often turn the scanner on while driving to IND to watch the traffic, but while I'm in motion it is extremely choppy. When I speed up it gets worse, and vice versa. It's very hard to make out transmissions.
I have noticed though that it's fine when I listen to it while flying. I don't believe it's bumps in the roads that make this happen. It seems more like something having to do with being in motion, but like I said it works in aircraft. Would a regular scanner as opposed to a handheld not do this?
As for finding freqs, before you leave I'd suggest finding a good freq and just stay on that. Then just look for another one when that one's no longer picking up ATC. I would urge you to pull over or have a passenger change freqs for you. Doing this while driving could easily cause an accident, especially on an interstate like I-70.
Hope this helps!