fuel consumption while idle is quite negligible particularly when it comes to a MPG calculation. My car (and actually all european models over the last few years) have auto-idle when stopped. My stats are that over the 10,000 km I have driven since new I have extended my range by 85 km in terms of saved fuel (so approximately 4 litres saved over 10,000 km, or approximately 4/700 (700l being total fuel comsumption over those 10,000 km).

So i've saved half a percent of my total fuel consumption. Useless individually. But of course potentially enormous savings when rolled out to a fleet of thousands of cars