I don't tend to travel a lot and whenever I have travelled some distance it has usually been in a car and I have once or twice fallen asleep during the journey mainly so that I can get through the journey without suffering from motion sickness but also so that the journey time passes a little faster as well whilst we are in the car. 

When it comes to travelling on something such as a train or plane, I feel the only way I would be able to sleep is if it were quiet or a very long journey.