The U.S. trucking industry’s demand for diesel is expected to weaken for the third year in a row as an economic slowdown reduces the amount of freight hurtling across the nation’s highways.

According to Reuters, freight trucks account for more than half of U.S. demand for diesel, meaning a slowdown in the industry could have a significant impact on overall fuel use, energy analysts said.

“We think that growth will be much more modest in 2008,” said Andrew Reed, oil market analyst for Energy Security Analysis Inc in Boston. “Trucking activity is a key driver of that forecast.”

Energy demand in the United States has already shown signs of weakness as high prices and the fallout of the housing crunch and credit crisis slow purchases at the pumps, leading to lower profits for U.S. refining companies, Reuters said.

Economists and energy experts also weighed in on the weak U.S. dollar, saying this, along with economic turmoil, could translate into lower imports of goods to ports and therefore less demand for road freight to move them.

“We expect tonnage to be lower, especially in the first half of the year,” said Bob Costello, chief economist for the American Trucking Associations.

The ATA’s tonnage index, tracking freight moved by the industry group’s members, declined 1.4 percent in 2007, after a 1.7 percent drop in 2006. That compares with a 12 percent rise during the 2002-2005 period, which was fueled by high imports and a construction boom, Reuters reported.

According to the ATA, trucking represents nearly 70 percent of tonnage carried in the United States.  Freight trucks accounted for about 52.4 percent of total diesel consumption in 2007, according to the U.S. Energy Information Administration.