Why is it sometimes very warm on board the vehicles?

The air conditioner will get worse when the doors are opened often, as they do on city buses. On the other hand, the heat, of course, not be turned on if it is warm. Passengers can point out to the driver that it is warm, because the driver does not always have time to go out in the bus at the end stops and know whether it is hot or not. The driver will then have a reasonable chance to fix it by, for example, turn off the heat and open roof hatches. There are no drivers who want the passengers to sit in uncomfortable heat by having the heat on.