Some facts:

• vehicle idling wastes about 3 billion gallons of fuel each year in the U.S. and generates around 30 million tons of carbon dioxide annually.
• Some drivers prefer to let the engine idle for 20 minutes or longer to get everything—including the cabin—really warm, but the fastest way to warm up an engine is by driving
• Cars newer than ~1995 are fully lubricated within 30 seconds of starting.

Some opinions:

• Comfort and convenience is the only reason to idle-warm a post-1995 non-diesel ICE car.
• The reckless proliferation of cars is due to the comfort and convenience they offer to addicts of comfort and convenience. Anything to reduce the comfort and convenience of cars is in itself ecological.

So new rule:

• If someone is idling a non-diesel ICE younger than 1995 for more than 30 seconds, they get a fine equal to the local value of 25 Big Macs (e.g. ~$250 in the U.S.), or 25 hours of community service.