I'm known to be car sick a lot. For some reason it never happens on buses/coaches, though. But on my way to college every morning...ugh. Also, the time of the day seems to be a factor as well, and traffic. If the car has to stop/move every few minutes it gets kinda bad. And it's bad when it's really early in the morning or around midday, don't remember getting sick at any other times.
Edit: And yes, at any time of the day if I attempt to read or play a portable game in a car I'd get really sick.
One time we had to stop at the nearest pharmacy and get pills for travel sickness.