I’ve already touched on this topic on the sister SE site once or twice. To summarize what is told there (check for more detail), there would be several considerations as related to energy use and sustainability:
- Idling in itself wastes fuel for no good reason, and that adds up. In US (country whose residents drive probably more than anyone else on Earth, to put it in perspective) the average cars idles 16 minutes a day (including warming up, waiting, and sitting in traffic). Even 2.0 L compact car would burn (at a rate of 1 L per hour) almost 100 L of fuel a year. Obviously, some idling is unavoidable, but even half of that saved and multiplied by the number of all the cars in the world would amount to significant impact.
- Idling negatively affects the engine internals due to incomplete combustion (glazing of the combustion chamber, fouling of the spark plugs) which further decreases fuel efficiency for all other regimes of operation.
- What’s more, excessive idling affects other components (burning up of the catalytic converter, increased corrosion of exhaust system, increased engine wear due to higher vibration etc.), whose premature wear and replacement lead to increased use of raw materials.
What one can do (and what I do) is to turn off engine during long traffic lights, railway crossings and so forth. Anything more than 10 to 15 seconds (very generously) would save fuel, because car that is already warm does not need that much effort to re-start, as it is at the proper operating temperature, and oil is distributed around the engine and at a high lubricity. Motor stays within the efficient range of temperatures for more than few minutes, and takes several hours to completely cool down (depending on the weather), so there is definitely no need to keep the car running during short errand (never mind someone could simply run off with the vehicle).
Any concerns about premature wear of the starting infrastructure are moot considering savings on fuel and other engine components (one could afford to replace starters every 3-4 years at the worst). See the linked posts for detailed arithmetic. And any energy losses that occur during starting, and keeping the electrical loads going during the time the engine is off are minuscule compared to energy used by petrol combustion while idling. See relevant question at Physics.SE.