Not much actual fuel-saving benefit; massive potential problems if the battery goes flat
Potential Fuel Saving: Negligible
Annoying and pointless, except in absolute gridlock
The Federal Government offers this recommendation in itsGreen Vehicle Guide website: “By having the engine switched off, even for a short period, you will save more fuel than is lost … in restarting the engine. The net increased wear and tear from this practice is negligible.”
On the Calais V V6 we tested, one engine start appears to consume the same amount of fuel as it takes to idle the engine for 6.3 seconds. (You have to do a great many engine starts back-to-back, and an hour of action-packed idling with your peepers glued to the ‘fuel used’ display to uncover this information.)
Actuaries might consequently advise shutting down for any stop longer than 6.3 seconds. The only problem with this analysis is that you have to know you'll be stopped for at least that length of time, which is not always possible.
There are also some practical considerations. For starters, it costs only four cents per minute to idle the Calais with the lights, radio and air conditioning on – that’s the princely sum of $2.40 per hour, or slightly less than an espresso coffee for every hour spent idling. If stop-go traffic weren’t already reprehensible enough, try doing it with multiple re-starts (The process: select neutral, or 'P', crank engine, radio goes dead, fire, radio resumes, select ‘D’, etc.) while anticipating the start-up delay just to get moving in concert with the surrounding traffic.
Also, if via this laborious process you manage to flatten the battery in the middle of the Burnley Tunnel, the Gateway Bridge or The Sydney Harbour Bridge - just once - the traffic chaos you cause will undo all the environmental benefit stemming from switching the engine off for years to come. It will also be a very unpleasant experience.
Some modern cars feature engine 'stop/start' technology, which automates the engine-shutoff and re-starting process in traffic, and which also monitors battery condition along the way. It's probably best to leave switching the engine off in traffic to these automated systems instead of putting yourself through the angst for a negligible consequent saving. The one exception is in absolute gridlock, where the traffic is stopped and has been for some significant time. And even then, remember to turn your headlights off.