here's another one: it is dirtier to start/stop the motor frequently than to let it run for a couple of minutes. Some people seem to want to kill the motor instead of just letting it idle, under the false pretense that they save fuel. But in the end you probably burn more fuel and you certainly add more wear and tear to the motor.
Within reason, of course. Idling for 15 minutes isn't better than one on/off cycle.