Hey Mark,
Well the potential fuel savings are off-set by the disadvantages that tardis mentioned. It’s rare for me to even put my car in park, such as while waiting for a train to pass and it’s more rare than a blue moon for me to feel compelled to shut down the engine. Maybe I will if the thing is just barely crawling along, but again, it’s rare.
I used to have a pizza gig and I would almost always leave the engine running while at the customer’s front door.
Doing regular preventative maintenance on your car will go a much longer way toward being green.
Hope that helps.