Mine too Leto, I specifically put my sims car in the inventory to drive it on vacation, but they still took the motorbike, I have only had one sim use her sports car and that was in Egypt. That was funny looking.
Haha, that should really look funny! Wonder what happens if you could run out of gas in the middle of nowhere. Let's just walk home people, the whole dessert long! Whaha! By the way, that isn't possible though, even if you would not die by lack of water and food. The point is that you will walk in circles, because your left food steps a little les as far as your right foot, or vica versa. But the result is the same: on long distances, and I mean very long distances, like a return ticked desert, you will definatly walk in circles.
But that will maby be a little to much of topic. Pam, how did you force your sims to drive in cars in foreign country's? Or was it just the only transport agent your sims got at that moment?