The Prius will normally turn off the engine when the car is stopped, which, for my commute is frequently — many lights, and heavy traffic in some parts. But in the winter, until the car has warmed up, the Prius decides to keep the engine on. Idling in traffic isn’t good.
But, if I turn off the heater, off goes the engine. Ha!
My mileage this week has been back up to normal (a little lower than the normal 50+ MPG in winter — in cold climates they change the fuel mix in winter so cars will run properly, even if less efficiently).
So when the car is moving and the engine is on, I turn on the heater, while slowed or stopped, I turn it off. By 10 or 15 minutes the engine is hot enough that the engine will stop on its own. It takes a little longer to warm up the car, and sometimes you need to turn on the AC to prevent the windshield from fogging up, but otherwise, it’s a pretty good trick.