The 3.3% drop was the largest month-to-month contraction since August '06, taking the index to its lowest level since November. Additionally, tonnage contracted 0.2% compared with March '07, marking the first year-over-year decrease in the index in six months. According to ATA chief economist Bob Costello, the latest tonnage reading represents a significant setback for the industry. "I've been concerned that the recent run-up in tonnage might not be sustainable, and clearly March's figures confirmed that apprehension," he says.

While he previously forecast a mild recession for the overall economy during the first half the year, he speculates that last month's tonnage drop might mean the economic contraction could be more severe than anticipated. "Truck tonnage often leads both recoveries and recessions, and the latest contraction suggests the economy and trucking are not out of the woods yet," the economist observes. "Surging diesel and gasoline prices are weighing heavily on consumers, and since trucks haul virtually all consumer goods at some point in the supply chain, the industry is going to be significantly impacted both directly through higher diesel prices and indirectly through lower freight volumes."

Costello says trucking serves as a barometer of the US economy because it represents nearly 70% of tonnage carried by all modes of domestic freight transportation, including manufactured and retail goods.