You got me, since I've never played any of my games 'on the go' before, but if my family had a mini van or other larger vehicle and we were going on a long road trip and I WASN'T driving(which I'm usually roped into), I would've probably grabbed a screen for the PS2 or Xbox.
Although, I believe there was a rumor going round of a portable Xbox at one point, though if it's anything like the original Xbox I'd venture a guess that you'd need to lift some weights just to carry it around. Although, if they made a portable one, I might well buy it, since I play my Xbox frequently.