Keeping traffic moving does more than cool motorists' tempers. It keeps the air clean, experts say.

Studies by the state Bureau of Air Quality show that as a vehicle's speed increases, output of polluting emissions decreases.That doesn't mean, however, that speeding is good for the environment.

"If traffic is flowing at 25 mph that's better than racing to 45 mph then stopping," said Richard Valentine, air pollution control specialist with the Salt Lake City/County Health Department.

"Traffic congestion, not speed, is the key word. When there is congestion nothing is happening but cars generating pollution."

He said figures will always show pollution output highest when cars are idling because studies plot grams of emissions per mile. So, when the vehicle isn't moving (no miles), the graph will show emissions at highest levels.

But as the car moves, its output of pollution decreases, and not just according to the graph. Vehicles run more efficiently and emit less when moving, Valentine said.

The lowest output of hydrocarbons, carbon monoxide and nitrogen oxides is at about 45 mph.

Health officials have taken their studies to cities along the Wasatch Front and traffic engineers have responded, Valentine said.

Adjustments in speed limits and traffic signal timing improve traffic flow and have reduced carbon monoxide levels significantly in Salt Lake County, he said. Another contributing factor is older vehicles that spew out more pollution than today's cars are being replaced by newer, cleaner-running models.

Along major routes in Provo, speed limits have been raised 5 mph and traffic lights timed to keep traffic moving and emissions down in Utah County where violations of air-quality standards have occurred three to four times a year.This winter, air quality tests will be conducted to see if improving traffic flow can have an impact. "Until we have a few years (of data) in, it will be hard to know if it has helped reduce pollutants. It (reduced emissions) can be offset by a number of things" such as population growth increasing the car traffic, said Ralph Clegg, county supervisor of the vehicle emissions program.

Provo City traffic engineer Nick Jones said raising the speed limit has not conflicted with safety. Engineers only make speed adjustments on heavily traveled routes, not in residential areas where speeds are set artificially low.

But when the speed is set too low on principal roads, he said, more motorists are found breaking the limit because they drive at whatever speed feels comfortable.

"Many times when you increase the speed limit, traffic slows and comes more into compliance," Jones said.