Air pollution continues to be a problem along the Wasatch Front but automobiles are not solely to blame, a regional planning group says.

And building more roads will ease automobile pollution by lessening congestion and increasing vehicle speeds, according to the Wasatch Front Regional Council.The council's assessment of current pollution contributors and projection of 2020 patterns come at a time when the Utah Department of Transportation is widening 17 miles of I-15 in Salt Lake County.

UDOT hopes to further relieve traffic congestion by building the Legacy Parkway in south Davis County. Construction of the 13-mile, four-lane highway is opposed by numerous environmental groups but supported by the regional council.

Among the more than two dozen charts and graphs included in the council's report is an analysis of how automobile pollution rises when traffic is gridlocked.

"At a lower speed, you get a lot of emissions. At a higher speed, the emissions go down," said Will Jefferies, the regional council's executive director.

"People say, 'Why will building roads make the air cleaner instead of dirtier?' Because you're getting rid of congestion."

If the state and local governments stopped building roads now, Jefferies said, average speeds during peak hours in 2020 would hover around 17 mph. Building the roads and making improvements now planned would add another 10 mph to that speed by decreasing congestion, he said.

Jefferies said automobile emissions technology has improved so much, and will continue to advance, that vehicle emissions in 2020 will be a mere fraction of the pollution problem they were in 1968 and are today.

"Every time you get a new vehicle, you decrease the amount of emissions that you are going to create," Jefferies said. "And as this vehicle mix changes from year to year, we get the cleaner vehicles into the system and the total emissions go down.

"Don't blame air pollution on the car without thinking."

But Nina Dougherty, chairwoman of the Utah Chapter of the Sierra Club, takes exception with a number of the regional council's conclusions.

Dougherty agreed that vehicles emit more pollution when crawling along in traffic but said building new roads does not mean the air will be cleaner. More roads will encourage people to travel more often and for longer distances, she said, while increased congestion results in fewer people driving.

Dougherty noted cities that have attempted to build their way out of congestion -- like Houston and Phoenix -- also are among the nation's leaders in air pollution.

Dougherty conceded automobiles are polluting less but said sports utility vehicles and diesel-powered vehicles continue to be major polluters. And more people are driving sports utility vehicles, she said.

Ursula Trueman, director of the state Division of Air Quality, confirmed that emissions from vehicles are declining through improving technology.

"The bad news is that it seems every year we are driving more," Trueman said. "So, at some point in the future, increased driving will offset the decrease in emissions because of the cleaner technology."

The regional council's data shows that automobile emissions account for a small percentage of fine-particulate matter pollution and will contribute in an even less significant way by 2020. Industry is by far the biggest contributor to PM10 (tiny bits of soot or dust 10 microns in diameter) emissions, the regional council found.

Trueman said vehicles do contribute to particulate emissions, although she said Jefferies is right that taking cars off the road alone is not the answer to the PM10 problem.

"We certainly know if we're driving less, we're emitting less air pollution, no doubt about it," she said. "You're driving down the road, you're emitting air pollution and you're turning up dust."

Road dust is the second-largest contributor of fine-particulate emissions in Salt Lake County each year, according to the council.

The report used federal Environmental Protection Agency projection models.