The American Trucking Association's advanced seasonally adjusted for-hire truck tonnage index fell 0.7 percent in May.

This slight decrease follows a 1.1 percent decline in April, which was reported on May 22.

The latest drop lowered the index to 117.8 (2000=100), down from April's level of 118.7. Compared with May 2011, however, the index was 4.1 percent higher, which was the largest year-over-year increase since February 2012. Year to date, compared with the same period last year, tonnage was up 3.8 percent.

The index, which represents the change in tonnage actually hauled by the fleets before any seasonal adjustment, equaled 124.5 in May, which was 6.5 percent above the previous month.

"Two straight months of contractions is disappointing," ATA Chief Economist Bob Costello says. "The drops in tonnage are reflective of the broader economy, which has slowed."

He adds, "The good news is that the decrease in fuel prices will help support retail sales going forward, which is a big part of truck tonnage."

He also reiterated his prediction from last month that annualized tonnage growth should be in the 3 percent to 3.9 percent range this year.

As a negative, Costello says he's concerned about businesses sitting on cash instead of hiring more workers or spending it on capital, both of which would give the economy and tonnage a shot in the arm. He notes that employers are worried about Europe and the so-called U.S. fiscal cliff, which refers to the tax breaks that are due to expire and the automatic spending cuts that are scheduled to take effect at the end of this year.

Each month, the trucking association asks its membership the amount of tonnage each carrier hauled, including all types of freight. The indexes are calculated based on those responses. The sample includes an array of trucking companies, ranging from small fleets to multibillion-dollar carriers.

Trucking serves as a barometer of the U.S. economy, representing 67 percent of tonnage carried by all modes of domestic freight transportation, including manufactured and retail goods. Trucks hauled 9 billion tons of freight in 2010. Motor carriers collected $563.4 billion, or 81.2 percent, of total revenue earned by all transport modes.

The trucking association calculates the tonnage index based on surveys from its membership and has been doing so since the 1970s. This is a preliminary figure and subject to change in the final report issued around the 10th day of the month. The report includes month-to-month and year-over-year results, relevant economic comparisons, and key financial indicators.