My Great Pyrenees did this once. Was headed down the freeway with the cruise set at 78.
Minivan pulls up and says “Your dog jumped out about a mile back.”
Turns out he got the topper door open and took a leap.
We turned around at the next exit and headed North to find him. Stopped near another vehicle and they asked if we were looking for a white dog, which we were.
They had seen him run into the ditch on the east side of the freeway.
After a few minutes of calling for him, he came running over to us.
Took him to the vet and they put staples in his eyelid, and his knees, but no broken bones.
He was a tough mother fucker.
Sadly, he bit my daughter a few years later and he had to go away.