Search all of DEQ Main Menu Utah Department of Environmental Quality
Secondary Navigation

Idle Reduction During Winter Inversions


tail pipeIt's a familiar sight on winter mornings. Cars parked in driveways, idling ten or fifteen minutes to warm up the engine. Drivers idling at drive-thru windows waiting for a cup of coffee, while waiting to pick up or drop off passengers, or just to stay warm while parked. The emissions from these individual vehicles add up, particularly when the air quality is poor. Many people are unaware that reducing the time a vehicle spends idling during winter inversions can really make a difference in air quality.

Research indicates that the average person idles their car five to ten minutes a day. Cutting idle time makes sense during inversions, because reduced idling also reduces the emissions of nitrogen oxides (NOx) and volatile organic compounds (VOCs), both precursor gases for the formation of PM2.5 during inversions. While the emissions from one idling vehicle may not seem like much, when combined with the emissions from other vehicles, the cumulative effect can be significant.

Idling also increases fuel costs. Ten seconds of idling can use more fuel than turning off the engine and restarting it. A car idling for two minutes uses about the same amount of fuel it takes to drive one mile.
Test your knowledge about idling with this quick quiz:

Idling is good for your engine.

FALSE: Excessive idling can actually damage your engine components, including cylinders, spark plugs, and exhaust systems. Most car manufacturers believe that idling more than 30 seconds is not only unnecessary but actually unadvisable.

Idling wastes fuel, costs money, and harms air quality.

TRUE: Idling wastes an enormous amount of money because it burns fuel that isn't used to drive the vehicle. A car that is idling gets zero mpg. Idling also harms air quality by producing exhaust emissions that emit fine particulates into the air and increase the formation of ozone.

It is necessary to idle your car on cold winter days.

FALSE: Many components of the vehicle—including the wheel bearings, tires and suspension system—will warm up only when the vehicle is moving. You need to idle no more than 30 seconds to get the oil circulating through the engine.

It is a good practice to shut off the engine when your vehicle is stopped for more than:

  1. 10 seconds
  2. 10 minutes
  3. 30 minutes

a: More than 10 seconds of idling can use more fuel than turning off the engine and restarting it. If you are stopped for more than 10 seconds, you'll save fuel and money by turning off the vehicle and then restarting it when you're ready to drive away.

I should turn my vehicle off when I am caught in stop-and-go traffic or at a long stoplight.

FALSE: The 10-second rule is a good one, but you can't avoid all idling. Turning off your vehicle in these situations might disrupt traffic. It could also cause you to speed away after you restart the engine, which would offset any fuel savings and increase emissions.

I need to idle my car on cold and hot days to stay comfortable.

FALSE: Idling on a warm day with the air conditioning on burns even more fuel than idling without the air conditioner. Opening a window in the summer or putting on warm clothes in the winter can reduce the need for idling. Alternative technologies to idling can provide comfortable conditions for drivers of heavy-duty vehicles.

Which of the following are common reasons for idling?

  1. Warming up a vehicle.
  2. Sitting in the drive-through lane of a fast-food restaurant.
  3. Stopping to talk to a friend.
  4. Waiting for someone.
  5. All of the above.
  6. All of these are all common reasons for idling and all of them can be avoided.

Restarting my car many times, rather than letting it idle, is hard on the starter and other parts.

FALSE: Studies show that restarting the engine many times has little impact on components such as the battery and the starter motor. Component wear caused by restarting the engine is estimated to add $10 per year to the cost of driving, money keysthat will likely be recovered several times over in fuel savings from reduced idling.


  1. If you are going to stop your car for more than 10 seconds, turn off the engine. When you start your engine, do not step down on the accelerator, simply turn the key to start.
  2. Rather than using a drive-through window, park your car, walk inside, do your business and then return to your car.
  3. Warm your car in the winter by driving rather than idling.
  4. In colder areas, use an engine block heater to warm the engine. This improves fuel efficiency and reduces emissions.
  5. Use remote starters wisely. Don't turn your vehicle on before you are ready to leave.

Information courtesy of Idle Free Utah, California Energy Center, and Natural Resources Canada.