Ford and BMW are on record stating that the stop/start break even period is 3-5 seconds.
The safety aspect is a dud. There's a red light/ stopped cars in front of you anyway - where are you possibly going to go?
On a factory S/S system, the engine will be running before you can put it in gear.
MPGuino can log fuel used idling:
That's 4.52 litres out of 55.65 for the tank or close enough to 10%. A non eco driver spends more time stationary so would waste even more idling.