An important debate is taking place over the value of transit in easing traffic congestion. This study sought to quantify the effect of light rail transit (LRT) on traffic in a travel corridor and provide quantitative data that can be used to shape future transportation policies aimed at reducing traffic congestion, energy consumption, and air pollution. Using a quasi-experiment design and data before and after the University of Utah’s TRAX LRT line was opened, we estimated that traffic on the street with LRT (400/500 South) decreased by 7,500 to 21,700 due to the availability of a high-quality transit serving destinations along the line, and, most important, the University of Utah. Traffic on 400/500 South decreased despite significant development in the corridor and expansion of the university. Based on our estimates, LRT along 400/500 South saves about 362,000 gallons of gasoline and prevents about 7 million pounds of CO2 from being emitted each year.