From Union of Concerned Scientists (emphasis added):

Victory on Reducing Diesel Pollution From Sleeping Truckers! Thanks to thousands of emails from UCS activists like you, and support from other residents across the state, the California Air Resources Board voted unanimously to set a 5-minute idling limit for diesel trucks that typically idle overnight! Trucking companies will now equip their sleeper cabs with cleaner, smaller engines or non-diesel alternatives to power air conditioning, heating, and other amenities for sleeping truckers. Thanks to this new rule, California will save over 100 million gallons of diesel fuel a year, prevent more than 700,000 tons of global warming emissions and thousands of tons of toxic and smog-forming air pollution from being emitted every year, and save truckers a small fortune in fuel costs.