CHICAGO For the second consecutive year, intercity bus traffic in the United States increased at a near-double digit rate thanks to a variety of factors, ranging from new service to the surge in gasoline prices earlier in the year.

According to a study from DePaul Universitys Chaddick Institute for Metropolitan Development, scheduled U.S. intercity bus departures grew 9.8 percent between the fourth quarters of 2007 and 2008.

This marks the second year of robust growth after more than four decades of persistent decline, said the institute. The annualized rate of growth between the second quarter of 2006 and the fourth quarter 2007 was 8.1 percent.

"Driven by downward economic pressures, spiking gas prices and a contraction in the nations airline industry, the intercity bus industry this year has posted its biggest one-year gain in service in a half century," the study said.

"The growth in intercity bus traffic is being driven by a number of factors, chiefly the spiking price of fuel over the past year," said Joseph Schwieterman, professor of public service and director of the Chaddick Institute, who headed the study.

"The basic economic efficiencies of bus travel are proving to be extremely attractive in this difficult economic climate."

Most of the growth in bus service was driven by increases in service offered by two companies megabus.com and BoltBus (a joint venture of Greyhound and Peter Pan bus lines).

The study noted that intercity bus service is most popular between cities 175 to 300 miles apart.

The study also pointed out that due to the significantly higher fuel efficiency per passenger mile achieved through bus travel, the trend toward buses for city-to-city travel produced a bonanza in reduced greenhouse gas emissions.

Approximately 36,000 tons of carbon dioxide were kept out of the atmosphere this year as a result of the shift, according to estimates compiled for the study.

While intercity bus service was growing during the year, other forms of long-distance travel experienced declines - especially air travel which has seen a roughly 8 percent decline seat-miles provided between the fourth quarters of 2007 and 2008.

Travel by private vehicle was down 3.3 percent for the first eight months of 2008, compared to the year earlier period.

Train ridership, as reported by Amtrak, however, was up over last year. The number of seat-miles on trains provided so far this year has increased about 3.3 percent. The majority of train service growth during the past year occurred in the Northeast, especially between New York and Washington, D.C.