A monthly index that gauges economic activity by tracking truck drivers' fuel purchases fell in September, marking the second consecutive monthly decline and raising concerns about the pace of shipping and economic activity in the United States.

The Ceridian-UCLA Pulse of Commerce Index (PCI), published by the university's Anderson School of Management, analyzes data from fuel credit cards swiped by drivers as they fill their rigs. The database, created by Ceridian, captures and analyzes the location and volume of fuel being purchased. Because the data is tracked in real time, UCLA and Ceridian said the index paints an accurate picture of product flow across the United States and by extension, overall economic activity.

The index declined 0.5 percent in September after a 1-percent decline in August. Taken together, August and September represented the biggest combined two-month decline since the trough of the recession in January and February of 2009.

The September 2010 data represented a 5.8-percent gain over September 2009. However, the monthly pace of year-over-year increases has been steadily declining since the 9-percent year-over-year gain recorded in May.

"The PCI tells us that inventory is stalled on the nation's thoroughfares. The good months of growth are now seemingly in our rear-view mirror," said Ed Leamer, director of the UCLA Anderson forecast and chief PCI economist, in a statement.

Despite data he called "alarming" for the U.S. economy's near-term prospects, Leamer does not foresee a relapse into recession. That's because so many economic indicators are already at or near record lows relative to growth of the nation's gross domestic product, he said.

The September PCI reading does not bode well for reducing the nation's unemployment rate, currently at 9.6 percent. PCI results need to reach the 10- to 15-percent year-over-year growth range to spur job creation. That is well above the September's year-over-year growth of 5.8 percent.

PCI economists said the October index, which will reflect activity during the trucking industry's peak period, will be an important window on how well transportation and the broader economy are likely to perform in the months ahead.

The PCI data provides more evidence of a slowing in the pace of shipping during the second half of the year. Bob Costello, chief economist of the American Trucking Associations, has forecast a moderation of truck traffic through the rest of the year. On Wednesday, JPMorgan Chase lowered its third-quarter estimates for trucker Con-way Inc., saying the company will face challenges in "quickly re-pricing a book [of] business that has a significant component of poorly priced freight." The analysts also said Con-way's September tonnage will only increase by the "low single digits" compared with the same period in 2009. By contrast, July's year-over-year tonnage growth stood at 16 percent, with August's tonnage growth coming in at 9 percent year over year.