Drivers who leave their cars running might consider keeping a stopwatch with them.

If they are inside Park City, there is now a three-minute limit on idling.

The Park City Council on Thursday night enacted a so-called anti-idling law in what leaders see as an important step in City Hall’s overall environmental programs. It is now unlawful for people to idle their vehicles for longer than the three-minute limit. First offenses will carry a warning while subsequent ones will be punishable with a $100 ticket.

There was little discussion prior to the unanimous vote by the City Council. City Hall has for a while indicated it was interested in a law, and there had been a nonbinding resolution in place encouraging people not to idle their vehicles. There had already been a series of signs posted discouraging the practice.

“I better not catch you in violation of this ordinance,” City Councilwoman Liza Simpson told staffers in the audience, speaking in a friendly tone of voice.

One of the leaders of a group called Clean Air Park City, Mary Jacquin, told the elected officials her group supports the law. She said the Park City law sets an example for the rest of Utah.

Andy Beerman, who is with the Historic Park City Association, a group that represents businesses on or near Main Street, also testified in favor of the law. Vehicles are commonly seen idling on Main Street and Swede Alley. Beerman said reducing pollution could enhance Main Street’s image.

The City Council agreed to a set of circumstances when idling will be allowed, and emergency vehicles are exempt from the law when they are being driven on duty. Some of the other exemptions include when a vehicle is stuck in traffic and when the outside temperature is either below 32 degrees or above 90 degrees. Idling will also be allowed when someone is using a defroster to clear the windows.

The law also does not apply to vehicles that require the engine to be running to power equipment and systems like refrigeration units. People who are inspecting or repairing a vehicle will be exempted as well.

Parking-enforcement officers and police officers will enforce the law. Phil Kirk, a Park City Police Department captain who attended the Thursday meeting, said afterward the police have received complaints about idling vehicles. He said regular Parkites may continue make reports to the police.

“Starting now, we’ll be looking at warnings,” Kirk said.

The law seemed to win support throughout Park City’s various interest groups, and there was not organized opposition. A City Hall official has said notable backers included the Park City School District, Park City Mountain Resort and Deer Valley Resort.

People who wanted a law against idling argued that the pollution from the exhaust is unhealthy and the emissions have bad effects on the quality of the air.