I can't remeber exactly who did this (but I think it was VW). The had a small car (a Lupo IIRC) with automatic gear switching running on a small and efficient diesel engine (something like 3 litres per 100km). They did three tests:

1. Let the engine idle at stops.
2. Automatically stop the engine when the car stood. Restarts where also automatic on first touch of the gas pedal.
3. Automatically stop the engine when slowing down (actually whenever the brake was used), restart automatically when releasing the brake (if not standing) or when gas pedal is touched.
   

Restarting the engine took only about 0.5 seconds. Well the outcome was like this: Stopping the engine whenever the car stood (which includes traffic lights, ordering at drive-throughs, traffic jam etc.) caused the amount of fuel need to drop by about 10% (almost completely independent of the driver). Stopping the engine whenever the brake was used dropped it by another 5-10%, depending on the driver.
Most experts at that time agreed that higher consumption engines might have even bigger savings. IMO its a pity that technology didn't make it into everydays cars.

cu,
sven