Final answer:
You should never send text messages while driving a bus under any circumstances, as it can lead to dangerous driving conditions and is against the law.
Step-by-step explanation:
The circumstances under which you can send text messages while driving a bus are none. It is unsafe and illegal in most jurisdictions to text while driving, as it significantly distracts the driver and increases the risk of an accident. This applies even when the bus is at a red light or moving at a low speed—a driver's attention should always be focused on the road and on operating the vehicle safely.