Why do most Buses and Trucks keep their engines idling rather than shutting them off while waiting for passengers or cargo?

The key to the answer is the fuel used in the bus or truck. If you observe carefully, you'll notice that the "idlers" are diesel-powered vehicles. We always thought that bus drivers were leaving engines idle out of laziness but these diesel-powered engines require a certain level of heat to operate most efficiently. It is cheaper to leave them running than cold starting. Diesel fuel will last last almost twice as long as gasoline when used under the same atmospheric conditions.
Idling can also be a safety issue. Most buses and many big trucks, operate with air brakes. Air brakes can't operate effectively until sufficent air pressure has built up, a process that can sometimes take up to ten minutes.
And one issue pertains specifically to school or public buses--comfort. Bus riders want and expect comfort--air conditioning in the summer and heat in the winter. Buses have a massive amount of air space that takes a longer period of time to heat or cool than an individual's automobile.http://www.eurekalert.org/pub_releases/2010-03/naos-ife0110.php