Idle reduction is type of automobile emissions control aimed at reducing the amount of energy wasted by an idling vehicle. When a vehicle's engine is not being used to move the vehicle, it can be shut off entirely—thereby conserving fuel and reducing emissions—while other functions like accessories and lighting are powered by an electrical source other than the alternator. Each year, long-duration idling of truck and locomotive engines emits 11 million tons of carbon dioxide, 200,000 tons of oxides of nitrogen, and 5,000 tons of particulate matter into the air.
Idle reduction is particularly significant for vehicles in heavy traffic and trucks at the estimated 5,000 truck stops in the US. Many hybrid electric vehicles employ idle reduction to achieve better fuel economy in traffic. America's fleet of around 500,000 long-haul trucks consumes over a billion gallons (3.8×109 l; 830 million imp gal) of diesel fuel per year. Services such as IdleAire and Shorepower provide power at truck stops to resting truckers who would otherwise need to continue idling during mandatory breaks. Because the United States Department of Transportation mandates that truckers rest for 10 hours after driving for 11 hours, truckers might park at truck stops for several hours. Often they idle their engines during this rest time to provide their sleeper compartments with air conditioning or heating or to run electrical appliances such as refrigerators or televisions. There are other technologies that can reduce the use of fuel to heat or cool the cab when the vehicle is traditionally idling overnight. These can be battery or fuel powered but in either case, use less fuel, do no harm to the vehicle's engine, and add far fewer or even no additional emissions into the atmosphere.