Heavy-Duty Vehicles

Overnight Idling

Anyone who has driven on interstate highways knows that tractor-trailer trucks usually idle overnight while their drivers sleep. The trucks are kept running to

• Heat and cool the cab and sleeper,
• Mask noises,
• Keep the fuel warm in winter,
• Avoid cold starting, and
• Provide for personal safety.

Long-haul trucks typically idle 6 hours per day, or 1,830 hours per year, but actual practice varies, from idling 1-2 nights per week to hardly ever turning the engine off. Locomotives, buses, freighters, and cruise ships idle during overnight stops for similar amounts of time.

Workday Idling

Heavy-duty vehicles idle during the workday for a variety of reasons. These can include driver and passenger comfort, engine warmth, and the need to power electronic equipment or work machinery. Buses, locomotives, and marine vehicles can idle for similar reasons.

Light- and Medium-Duty Vehicles

Workday Idling

Idling fuel consumption with respect to time. (For reference, the Honda Civic engine is a 1.8 liter, the Ford Fusion a 2.5 liter, and the Chevrolet Malibu a 3.6 liter.)

Light-duty vehicles include passenger cars, livery vehicles such as taxi cabs and limousines, pickup trucks/small vans, and police cruisers. These vehicles typically idle for passenger comfort, engine warmth, and to provide power for accessories and electronic equipment.

Medium-duty trucks encompass courier and package delivery trucks and utility trucks, and they typically idle for the same reasons as light-duty vehicles. Medium-duty truck idling may be of short or long duration, depending on the reason(s) for it. In some cases, these vehicles idle to support power take-off for utility equipment such as lift buckets, pumps, or lighting being used by crews in the air or underground..

Fuel Consumption and Emissions

Each year, U.S. passenger vehicles, light trucks, medium-duty trucks, and heavy-duty vehicles consume more than 6 billion gallons of diesel fuel and gasoline combined—without even moving. Roughly half of that fuel is wasted by passenger vehicles (cars and light trucks); the remaining half by medium- and heavy-duty vehicles. As shown in the chart above, the bigger the vehicle’s engine, the more fuel it consumes when idling.

In addition, idling vehicles emit particulates (PM_2.5 and PM₁₀), nitrogen oxides (NO_x), carbon monoxide (CO), and carbon dioxide (CO₂). These emissions, along with noise from idling vehicles, have led to many local and state restrictions on idling.