On the trail the dog can ride wherever she wants too, depending upon how many other passengers there are.
On the road she is either crated or teathered. Either way, she's tied down so she doesn't become a doggy missle in an accident. I can secure the crate or she has a harness which can be tied off to any number of places, depending upon where she is sitting in the vehicle.
Unrestrained stuff is very dangerous in an accident. I'd hate to think what a 75 lb. dog would do to the back of my head when she flies through the vehicle.