Is a bus driver permitted to drive with one of its doors open?

1. It is only permitted when the air-conditioning system malfunctions.
2. It is only permitted when there are no passengers on the bus.
3. It is only permitted when approaching a level crossing.
4. Correct answer: Driving with an open door is forbidden.

WhyA bus may not drive with a door open. Not even empty, and not even if the air conditioning is broken.