For most conventional vehicles (i.e. not hybrids), idling longer than about 30 seconds (or even less – many experts cite 10 seconds) uses more fuel and thus produces more emissions than restarting.
It’s not practical to be turning your engine on and off all the time in traffic, but there are plenty of other situations where unnecessary idling happens e.g. waiting to pick up passengers, at fast-food drive-throughs, and talking on your cell phone*.
Modern engines also only need a few seconds to warm up, certainly not more than 30; “warming up” the engine for minutes at a time is an outdated throwback to earlier automotive times.
Frequent restarting does not do significant damage to vehicle mechanics; excessive idling, on the other hand, can eventually lower engine performance.
(Olivia Rose-Innes, EnviroHealth Editor, Health24, September 2011)
*There are other good health and safety reasons to shun drive-throughs and phoning while driving, of course...