Originally Posted by eepeqez
The average trip length for urban vehicles is something like 10km/6miles; the average traffic light interval is probably more like 0.5km/0.3miles. Restarting at every second traffic light instead of just once per trip has the potential to reduce the life of starter motors and ring gears by a factor of ten, bringing your hypothetical 20 year starter life back to just 2 years.
Automated engine off and restart at stops in traffic lights has only become viable with larger batteries and starters.
Mazda has a system for their direct-injection engines that doesn't require using the starter motor at all. The engine is stopped with a piston just past TDC ready for the power stroke. To restart, fuel is injected and sparked. Off you go.
To be fair, idling a modern 4-banger for an hour uses a ridiculously small amount of fuel. You'd have a bigger benefit from reducing aero drag by slapping on an underbody tray or finding a way to take 100lbs out somewhere.