[Andyman]

What I think is that it's usually not worth doing. Your car probably burns about 0.25 gal/hr of gasoline when idling. That translates to about 1.5 cents per minute if you pay $3.60 per gallon. Each time you stop your engine you will have to use the starter motor again except at the end of your trip. I estimate that replacing the starter costs $300 if you include a possible tow charge and lost time at work, and it last for about 10,000 starts until it needs replacement. This would mean that each start costs 3 cents. If these estimates are accurate, you would need to keep the engine off more than two minutes to save any money. When the engine is cold and running at a fast idle, it burns more gasoline so it makes more sense to shut it off if you have to stop for a while.

If you could use a small electric pump for the transmission fluid, the situation would improve because you could start the engine again by shifting into gear (with the car moving at sufficient speed) instead of operating the starter motor.