Will fast freeway speeds mean more pollution?

For at least a year, Utah air-quality activists have urged slower freeway speeds during the Wasatch Front's inversion and ozone seasons.

But instead of ratcheting down the state's speed limits, the Utah Department of Transportation sent crews out this week to replace 65 mph signs on Interstate 15 with new 70 mph placards.

Advocates say the higher freeway speeds are taking effect without much thought for their impact on the Salt Lake Valley's summertime ozone and wintertime particulate pollution loads.

"They could help reduce our emissions levels by decreasing the speed limit in our urban corridors to 55 or 60 mph," said Reuben Wolsey, a Salt Lake City resident who blasted the increased speed limit at the Air Quality Board's meeting last week. "There is precedent in other Western metropolitan areas for doing so, and significant scientific testing that shows that these lower speeds reduce emissions and allow more vehicles to travel safely and steadily."

Utah Moms for Clean Air highlighted lower freeway speeds at a rally last winter at the state Capitol that drew 5,000 Utahns, many wearing surgical masks.

"There is a lot of contradictory information," said Ingrid Griffee, executive director of Utah Moms. But "why take the risk in the face of conflicting information on emissions and traffic safety? What benefit could equal out that risk?"

The 2014 Legislature authorized UDOT to increase freeway speed limits as long as the change "is based on a highway traffic engineering and safety study."

The Utah Division of Air Quality (DAQ) weighed Moms' proposal for reduced speed limits and concluded slower driving won't help much. And conversely, faster freeway speeds won't hurt air quality either, according to Glade Sowards, a DAQ environmental engineer who studies mobile pollution sources.

But critics reason that faster driving cuts into fuel economy, so doesn't it follow that increased fuel consumption results in greater emissions?

State air-quality regulators say no. Today's catalytic converters are so efficient, they catch 90 percent of most pollutants once the engine is warm, so there no longer is much connection between fuel economy and emissions for the pollutants that degrade air quality.

"Most of the emissions over the engine's duty cycle occur in the first 50 seconds it is running," Sowards said. By the time a motorist gets on the freeway, "you have already done your damage for the day."

In a legislative presentation early this year, air-quality monitors reported tailpipe emissions of three major pollutants don't change much when vehicle speeds increases from 55 to 75 mph. Traffic congestion — stopping and going repeatedly — plays a far larger role in emissions than overall travel speeds, they said.

"Repeated acceleration and deceleration cycles increase all sources of emissions and decrease fuel economy," DAQ said in its report.

Modeling shows a little more than 40 tons of nitrogen oxides a day will come off Salt Lake County's freeways during the winter, along with about 28 tons of volatile organic compounds and 2.4 tons of fine particulate, or PM2.5, pollution. These numbers show slight changes if speeds change from 55 to 75 mph.

Utah Physicians for a Healthy Environment President Brian Moench says that state analysis examines just emissions from a tailpipe. It ignores others that may be just as important, including volatile organic compounds evaporating from a car's fuel delivery system and road dust kicked up by its wheels. Moench cites research that shows evaporative emissions of the compounds, precursors to ozone and fine particulate matter, increase with speed.

"We were blindsided by UDOT's announcement on Friday and we intend to fight this," he said. "We have contacted the governor's staff to ask that they intervene."

State highway managers, meanwhile, says most non-rush hour traffic on I-15 has been moving at speeds in excess of 70 mph.

"We don't believe changing the speed limit will directly correlate into increased speed by the traveling public," UDOT operations chief Jason Davis said. "It's more a reflection of what the public is already driving."

The Utah Highway Patrol has pledged strict enforcement of the new limits. Rural limits already have been raised to 80 mph, yet actual speeds have not increased much, UDOT data show.

"People drive the speeds they are comfortable with," Davis said. But he agrees road dust is a problem.

"We sweep I-15 every day — at least part of it," he said. "Anything we could do to ensure traffic moves smoothly will improve air quality."

Air-quality advocates remain unconvinced and point to other states that used lower speed limits to control vehicle emissions.

In 2006, Tennessee officials reduced the freeway speed limit to 65 mph around Knoxville as a way to get the Eastern Tennessee Valley into compliance with federal ozone standards. The speed limit for trucks was lowered to 55 mph.

"A study conducted by the Federal Highway Administration shows that reducing truck speed limits by 10 mph can reduce the nitrogen oxide emission factor by approximately 18 percent or more per truck," the Tennessee Department of Transportation said in a news release. "If we get reductions anywhere close to that level, it will be worth it."

Utah's higher speed limits are outliers in the Intermountain West, where neighboring states, with the exception of Wyoming, cap highway speeds at 65 mph, according to the National Motorists Association.

Whether Utah drivers and freeway managers agree with the emission science, there's no doubt driving faster will burn more gasoline.

"It's a double-edged sword. The faster you go, the more fuel you use. The more fuel you use, the more our local [oil] refineries create to keep up with the demand," Wolsey said. "They in turn refine more, pollute more, and what they emit is much nastier than the combusted fuel byproduct that leaves our tailpipes."

bmaffly@sltrib.com