I mostly drive alone and the wife follows in the car. She loves her relaxing music and sometimes will play educational cd's.
I will put pre-copied songs in my SD drive, that I listen to when the radio gets boring.
I prefer listening to the local radio to any other format due to the area I am traveling in information.
Usualy will search for a Oldies/Country station and go with it.
I find that silence is so golden in heavy trafic, as I can keep my attention an the road.