Why must the driver turn off noise sources before listening at a railroad crossing?

An approaching train can often be heard before it is clearly seen, especially around curves or in fog. Engine, heater, AC, and student noise inside the bus can mask the sound of the train's horn or the rumble of approaching cars. Turning off the heater/AC fan, having students quiet down, and opening the door and window lets the driver listen as well as look. The procedure has saved lives at crossings with poor sight lines. Even with active flashing lights and gates, the driver verifies in person — equipment failures and timing issues have caused multiple fatal incidents historically.