If your car is idling, you're getting zero miles per gallon. Sucks, huh? I quote from Treehugger:
While older cars from the 1970s and early 1980s might have needed time to warm up, the norm since the mid-1980s has been fuel-injection vehicles which can be restarted frequently without engine damage. When you start a car’s engine, a little bit of extra gas is used to get the combustion process started. That means a good rule of thumb for conserving gas is to idle for no more than 30 seconds, except, of course, if you’re just sitting in traffic. If you have a hybrid car, it does the work of avoiding idling for you. While parked or sitting in traffic, hybrids shut off their gasoline engines automatically.
Turn 'em off folks.