In the latest sign of the slowing pace of U.S. economic activity, the American Trucking Associations (ATA) said Tuesday its truck tonnage index fell a seasonally adjusted 0.7 percent in May, following a 1.1-percent seasonally adjusted decline in April.

The May index was 4.1 percent higher than in May 2011, representing the largest year-over-year increase since February 2012, ATA said. Year to date, tonnage is up 3.8 percent over the first five months of 2011, the association said.

The not seasonally adjusted index, which represents the change in tonnage actually hauled by the fleets before any seasonal adjustments are made, equaled 124.5 in May, which was 6.5 percent above April's number.

While truck tonnage was expected to level off during early 2012 after surging at the end of 2011, a slowdown this far into the year has some experts concerned. Even Bob Costello, ATA's ever-optimistic chief economist, couldn't put a good face on the May data.

"Two straight months of contractions is disappointing," Costello said in a statement. "The drops in tonnage are reflective of the broader economy, which has slowed."

Costello said truckers should get a tailwind from the significant decline in oil prices, which should free up more of consumers' cash flow for retail spending, which in turn will boost demand for truck services.

However, Costello, like other economists, is worried that businesses will be reluctant to hire and spend for the balance of 2012 due to concerns over the global economy and the possibility that Bush-era tax cuts will be allowed to expire at the end of the year, leading to large tax increases that will stunt economic growth.

Costello said he stands by his earlier projection of a 3.0- to 3.9-percent increase in tonnage in 2012. According to ATA data, tonnage in 2011 rose 5.9 percent over 2010 figures, the biggest annual increase since 1998.