A stretch of less than a mile going speed limit of 80, you can do the alphabet. It does require one swivel to see a sign from the other direction to get Z.

Uh, I may have done it while driving. Talk about distracted driving. I wouldn't recommend doing it. This was with no vehicle sign rules.

Now, something else I noticed. If you get to use vehicle sign and license rules, you can get a lot of mileage out of the back of a 15 foot U-haul truck.