When they were alive I regularly drove with one dog at a time (out of 3). I'd strap a blanket to the seat so their nails didn't puncture it and strap a harness to the seat mount in the center, just so if a fast tactic was needed to avoid a traffic incident they'd be as safe as practical (and not fly out). The big guy at 125 lbs was a bit cramped on the seat though he still loved it. I didn't drive fast as bugs could hit them in they eyes due to the open cockpit. Freeways were avoided due to the rubber bits that fly off tires as that could cause eye infections.
The only thing that sucked was getting caught in the rain, the smaller dogs would tend to ball up in the footwell to stay warm, the big one was a tank.
A tall windscreen helps short dogs, but large dogs sit so tall it often doesn't much matter.