"One of the main justifications for building a light rail line is the hope that it will reduce traffic congestion in a corridor, presumably by drawing commuters out of their cars and onto the train," writes Jaffe. "When we last looked at this assumption, about a year ago, we found cautious support for the decongestive value of light rail corridors in Denver. While traffic continued to rise in these corridors, it rose even more in nearby areas without the rail."
"A similar new study of British light rail systems comes away far less hopeful. In an upcoming issue of the Journal of Transport Geography, planners Shin Lee and Martyn Senior of Cardiff University found that the evidence for light rail reducing car use is unclear. Lee and Senior discovered that car ownership and car commute share often continue to rise in these corridors, and that ridership growth is often the result of travelers shifting over from buses — not cars."
After studying the travel habits of those living within four UK light rail corridors completed from 1991 to 2001, compared to four control areas, "the researchers had a hard time concluding that the light rail systems, taken together, produced much of a shift away from car commuting."
"In the end," adds Jaffe, "the researchers caution against expecting major long-term reductions in road congestion after the creation of a light rail system."