A legislator saw a dog do just that, and introduced a law outlawing it. They are supposed to be tied in such a manner that it can't jump over the side. Mine rides about 4 miles back from the lake and I don't tie him, but since his injuries he can't jump out. Even before then he wouldn't... I drive the road in front of my property at about 10 mph with the tailgate down and he won't get out until I say that it's OK.
The safest place for a dog is in a crate in the back seat, but I've never had a dog which would stay in one.