This seems easy yet hard…

1. Build a turn around past the last spot to access the road on both ends.
2. Place automatic gate at the base by the turn around.
3. Install a camera that measures the vehicle at the gate. If it’s too long gate doesn’t open and message shows on one of the message panels that they must turn around and proceed down the hill. If vehicle is not too long the gate opens and they proceed.

Problem solved.