Estimate fuel wasted idling.
An idling engine achieves zero miles per gallon, so every minute is pure loss. Burn rates depend mostly on displacement: a small four-cylinder uses around 0.2 to 0.3 gallons an hour, a V6 about 0.5, and a large truck or an engine running air conditioning well over one. Twenty minutes a day at half a gallon an hour is 42 gallons a year — about 145 dollars and 820 lb of CO2. The practical rule from fuel economy research is that idling longer than about ten seconds uses more fuel than switching off and restarting; modern starters and batteries are designed for exactly this, which is why stop-start systems exist.
Idling Fuel Waste
Daily gallons = idling minutes / 60 x idle burn rate; annual = daily x driving days; cost = gallons x price; CO2 lb = gallons x 19.6
Only for a very short stop. Restarting costs roughly the equivalent of ten seconds of idling, so any wait longer than that is cheaper spent switched off.
No. Fuel-injected engines warm fastest under gentle load, and prolonged idling from cold both wastes fuel and dilutes the oil with unburnt fuel. Drive off after 30 seconds and take it easy.