Turning on the engine and letting your car warm up before driving away is a rite of winter in New Jersey.

It is also against the law, if done for too long. (Though police generally rely on warnings and education rather than handing out tickets.)

Sparta Police Lt. John Lamon said it is not uncommon for an officer to spot a parked vehicle left running while the driver dashes inside a convenience store.

Rather than hand out a ticket, Lamon said police will typically issue a warning and get a thankful, if slightly astonished, response.

“A lot of people don’t even know the law. They’ll go, ‘really?’”

New Jersey prohibits idling -- defined as when the engine is running but the vehicle is not moving -- to no more than 3 minutes under most circumstances, with tickets starting at $100.

To some, it might seem like a minor law, but it has the serious aim of safeguarding health and the environment. While idling in some cases is unavoidable -- the law does not, for example, apply to drivers stalled in traffic -- it is also the source of potentially deadly air contaminants.

Jeff Tittel, executive director of the New Jersey Sierra Club, said trucks and school buses are among the primary sources of the air pollution.

“New Jersey has some of the worst pollution in the nation,” he said.

Still, a motorist in violation of the law has relatively little risk of getting ticketed. In 2016, 288 summons were issued statewide, up from the 276 summons written in 2015, according to the state judiciary.

Statistics for 2017 were not immediately available.

Tracy Noble, a spokeswoman for AAA Mid-Atlantic, reiterated the organization’s guidance that idling a vehicle is unnecessary.

“Vehicles today warm up very quickly. Letting it warm up 15 to 30 seconds before driving away is all your car needs,” Noble said.

Potentially deadly carbon monoxide is always a concern with idling. Lamon said police are alert for any vehicles in semi-enclosed spaces, such as a parking garage.

Rob Jennings may be reached at rjennings@njadvancemedia.com. Follow him on Twitter @RobJenningsNJ. Find NJ.com on Facebook.