As the American Public Transportation Association triumphantly reported in March, mass transit ridership in the U.S. reached  its highest level since the 1950s last year. But do those crowded buses, subways and commuter trains reflect a long-term trend, or merely a transitory reaction to the price of gasoline?

The answer appears to be both: A newly published study finds “a small but statistically significant amount of ridership fluctuation is due to changes in gasoline prices.” But perhaps the most interesting findings in the report by geographer Bradley Lane of Indiana University is how the trends differ from one metropolitan area to the next.

For his study, published in the Journal of Transport Geography, Lane looked at transit ridership in nine major American cities from January 2002 to April 2008. Gasoline prices began rising dramatically in the middle of that period, in August 2005.

Lane found that in Boston, Chicago and Denver, increasing gas prices were associated with an increase in ridership. “Escalating gasoline costs appear to have reversed a trend of decreasing ridership in these cities,” he reports.

In Houston, Miami, San Francisco and Seattle, rising gas costs were also associated with increased ridership, but their impact was less clear because mass transit usage in those cities was increasing independent of pump prices

Lane notes that Boston and Chicago have extensive mass-transit networks, and Denver “has instituted several significant projects that are playing a role in expanding transit service coverage and encouraging transit-friendly, mixed-use development in the traditionally auto-oriented city.” When gas prices shot up, commuters in those areas took advantage of the excellent public transportation networks already in place.

“By comparison, Houston, Miami and Seattle have less extensive service coverage,” the study adds. As these relatively young cities mature, it is possible a transition is taking place toward more mass-transit use — one the high gasoline prices apparently accelerated.

But will those bus and train riders return to their cars when and if the price of gas goes back down? Lane notes that part of the answer is in the hands of the agencies that operate the transit systems.

Like individual drivers, “Transit agencies are also subject to the constraints of high fuel costs,” he notes. But he cautions against giving into the temptation to cut service to save money.

Rather, “Transit operators and policymakers might consider subsidizing an expansion of services and enhancing quality at times of high gas prices so as to keep their new patrons should gasoline prices fall again,” he writes. “The short-term deficit of doing so might lead to a greater long-term benefit, both for the transit operation and for any broader objective of changing travel behavior by reducing driving.”

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