Long highway trip: Progressive rock/metal, and some power metal (of the more symphonic/proggy variety): Dream Theater, Kamelot, Opeth, Symphony X, Evergrey, Porcupine Tree, Tristania, Queensryche, Yes, Ayreon, Angra, Nightwish, Epica, Rhapsody, etc...
The music varies a lot, remains interesting throughout, lasts forever, and will definitely keep you awake, but also entertained.
There's one exception, though. If you happen to be on a Southern US highway, driving a car/truck with at least 8 cylinders, there's only one real answer: ZZ Top
For city driving, you need something energetic and fast. My personal favorites include power metal (Rhapsody works for this too, Sonata Arctica, select Kamelot or Angra songs, Lost Horizon, etc...), melodic death metal (Arch Enemy, Dark Tranquility, Death), or straight-ahead rock or metal (Paul Gilbert, Armageddon, Iron Maiden, Racer X, Firewind, Dream Evil, Pantera...)
These threads pop up pretty often, but I always enjoy them
Modified by eibbor at 11:06 PM 11-15-2007