Dogs need regular breaks to stretch their legs, have a drink and also to answer the call of nature, just like we do.
I recommend you stop for 5 minutes every hour or every 2 at the most. Take the dog for a bit of a walk/run/sniff and have a short play session.
You could invest in a long lead which will let your dog wander a bit further while still being under your control.
-----Response from Vicki:
Thanks, Giovanni. It may be very difficult for professional truck drivers to stop that often for pet exercise.
Drivers, we all know that the more and longer you stop your trucks -- at least those of us who are paid by the mile -- the fewer miles that you can roll down the road and make money. Have you found that certain breeds of dogs are better travelers than others? Or have you found that certain breeds are better able to travel longer distances than others? If so, which ones?
For more tips on pet care, please see Giovanni's site: http://www.my-pet-care-directory.com/