This is an archived article that was published on in 2014, and information in the article may be outdated. It is provided only for personal research purposes and may not be reprinted.

A new University of Utah study shows, for the first time, that adding a light rail line along a busy street in a growing area may not merely hold down future traffic growth, but can even reduce volume to below initial levels over time.

The study looked at effects of TRAX on 400 and 500 South in Salt Lake City, and "is the first study to document important effects of light rail transit on traffic volumes," said Reid Ewing, a professor of civil and metropolitan planning and lead author.

"Since the University TRAX line opened, there has been increased development in the 400/500 South corridor, yet traffic on the street has actually declined," he said.

"Our calculations show that without the University TRAX line, there would be at least 9,300 more cars per day on 400/500 South, and possibly as many as 21,700 additional cars. The line avoids gridlock, as well as saves an additional 13 tons of toxic air pollutants" a year, Ewing said.

The study compared traffic, not only on 400 and 500 South but along nearby parallel roads, before and after the TRAX line was built. It also looked at neighborhood development. The study found that the area grew since TRAX was built, but road traffic did not.

"Instead, we found just the opposite. As the corridor became more developed over the decade — concurrent with the opening of TRAX — traffic actually declined," Ewing said.

While development along the corridor increased by 13 percent between 1999 and 2009, vehicle traffic volume decreased by an average 21,700 daily trips. Researchers said that saved 487,000 gallons of gasoline a year, and 9.5 million pounds of carbon-dioxide emissions.

Ewing said while that finding could be important, "We cannot guarantee that light rail would have the same effect on traffic at other locations" because of some unique characteristics of the 400/500 South corridor. Light rail there goes to the University of Utah, which is a major state employer and destination — and where faculty and students are given free passes to use TRAX and buses.

The report was issued recently by the National Institute for Transportation and Communities, and has been accepted for publication in the Journal of Public Transportation later this year.

Michael Allegra, president and CEO of the Utah Transit Agency, said the study helps show that TRAX to the university "has resulted in stabilized traffic flows, increased economic investment in the corridor and significant increases in transit ridership."