Intercity for-hire truck tonnage in December and for all of 2011 posted their largest year-over-year gains in 13 years, a sign of strong seasonal demand and better-than-expected inventory restocking activity by retailers, the American Trucking Associations (ATA) said today.

According to ATA data, tonnage in 2011 rose 5.9 percent over 2010 figures, the biggest annual increase since 1998. Tonnage in December rose, on a seasonally adjusted basis, by 10.5 percent over 2010 levels, the largest year-over-year gain since December 1998. On a sequential basis, December's tonnage rose 6.8 percent over November activity, the largest month-to-month gain in seven years, ATA said.

The not seasonally adjusted index, which reflects the change in tonnage actually hauled by fleets before any seasonal adjustments are factored in, hit 166.4 in December, up 0.8 percent from November's data, ATA said.

Bob Costello, ATA's chief economist, said although he expected to see tonnage gains in December due to seasonal holiday activity, he was surprised by the overall strength of the December data. The figures point to solid manufacturing output and better-than-expected inventory restocking activity, he said.

"Inventories, especially at the retail level, are exceedingly lean, and I suspect that tonnage was higher than expected as the supply chain did some restocking during the month," Costello said in a statement.

The nation's railroads may be experiencing a similar phenomenon. For the week ending Jan. 14, the railroads reported a 7.4-percent gain in intermodal volume over the same week in 2011, while carload originations of bulk commodities rose 5.5 percent over the year-earlier period, according to data from the Association of American Railroads.

An index of end-user inventory levels reported by the Institute for Supply Management (ISM) came in at 42.5 percent in December, 7.5 percentage points below November, when the index registered 50 percent. A reading below 50 percent indicates customers' inventories are considered too low. Customers' inventories have registered at or below 50 percent for 33 consecutive months, according to ISM, which publishes a widely followed monthly survey on manufacturing.