Interesting question. I've worked with autistic children and young adults for some time now and got feedback from parents as well as travel on long journeys myself with them so I have learnt that sometimes special measures are necessary.
1. Get to know before your journey if the child has any known triggers that may cause upset or disruption. For example, one young man I worked with needed to know some days in advance that we would be going on a car journey. We wrote this down for him in his journal/daybook so it was there in black and white. The date, time and who would be in the car with him were crucial.
2. Incentives to travel. Perhap if there is a purpose to the journey it is more understandable for an autistic person. You could point out or write down or have a photograph of a place or person you're going to visit. If the autistic person has a favourite item or toy they could take that with them for security.
3. Sequences. If you travel with us then you can go to the cinema later on. It is a fact that some autistic children cannot tolerate a break in routine but if you have to do it with a car journey then try to keep the routine intact by suggesting..... first the car journey.... then the routine.
4.Smart technology may help. Music or visuals or both to keep the mind relaxed. A favourite fun video or dvd can keep a child from stressing out.
5. Let them know that you will have a break during the journey if this is needed. Again write it down or have visuals available so that they know a structure is in place. So for example, - 30 minutes drive then a break for a drink or 1 hour drive then a break.
6. Have a strategy in place if the unexpected happens. If you're on public transport for instance you may want a suitably positioned seat so that it would be easier for you to acces an exit door or a quieter space or a toilet/washroom.
There are other factors to take into account - the behavioural patterns of the individual, medication issues possibly.
The basics are - to be as prepared as you can be before the journey and resourceful on the journey. Learn as much as you can and take the positives out of it in readiness for any future travel.