The decline continues a downward trend that began in November, a department report issued by the Federal Highway Administration said.

It is the first time miles driven on public roads declined in March since 1979.

It follows a "modest 1 billion mile" increase in the national odometer reading in February 2008 compared with February 2007.

The estimate comes from data collected at 4,000 automatic traffic recorders and includes an estimated reduction of 9 million tons of greenhouse emissions produced in the United States for the first quarter of 2008.

The decline also "underscores the challenges facing the Highway Trust Fund and its reliance on the federal gasoline excise tax," Acting Federal Highway Administrator Jim Ray said.