A new rule limiting heavy-duty trucks' idling time went into effect in North Carolina this week

. Under the rule, which was adopted in July 2009, operators cannot idle more than five consecutive minutes in any 60-minute period, except in safety, health and economic situations. The rule applies to on-road gasoline and diesel-powered vehicles over 10,000 pounds.

Trucks are allowed to idle if necessary for refrigerating, hoisting, loading and responding to emergencies. The rule also exempts farm and military trucks. Trucks with an occupied sleeper berth may also idle for the purposes of air conditioning or heating during federally mandated rest periods. This exemption will expire May 1, 2011. For a complete list of exemptions, click here.

The state adopted the rule as part of the its efforts to reduce air pollution to meet more stringent federal air quality standards.

"Exhaust from idling trucks is a significant source of air pollution that we can't ignore," said Sheila Holman, director of the N.C. Division of Air Quality. "The idle rule will help improve air quality while conserving large amounts of fuel."

According to the state Division of Air Quality, the rule should reduce nitrogen oxide emissions by up to 1,300 tons per year.

The Division of Air Quality can assess fines up to $20,000 per violation, but it does not expect to assess fines for a while, according to Tom Mather, public information officer. Initially, enforcement would be compliant-driven, he said, and they will most likely start out with warnings.

The Division acknowledges that the idle rule will increase costs for truck operators, but it says the rule will save up to 9 million gallons of fuel per year, offsetting drivers' expenses.

In addition to the idle rule, the state Division of Air Quality started a public awareness effort to reduce unnecessary idling voluntarily for cars and trucks. As part of that effort, the state agency is distributing free signs that urge drivers to reduce idling at schools and other areas.