Americans drove 100 billion fewer miles during the 12-month period between November 2007 and October 2008 compared with the prior year, according to the DOT's most recent data.
U.S. Transportation Secretary Mary Peters noted that driving continued to decline even as gas prices came off their summertime peaks.
"The fact that the trend persists even as gas prices are dropping confirms that America's travel habits are fundamentally changing," she said in a statement.
The nationwide average price for unleaded gas was $1.656 per gallon on Friday, according to the motorist group AAA. Gas prices hit a peak of $4.114 a gallon on July 17.
In October, driving volume posted its steepest monthly drop since 1971, according to the DOT. Americans drove 3.5% less, or 8.9 billion fewer miles, compared with October 2007.
Transportation officials said they were concerned the drop would sap funding for the nation's highway system.
"As driving decreases and vehicle fuel efficiency continues to improve, the long term viability of the Highway Trust Fund grows weaker," said Peters. "The way we finance America's transportation network must also change to address this new reality, because banking on the gas tax is no longer a sustainable option."
Federal Highway administrator Tom Madison said in a statement, "This underscores the need to change our policy so American infrastructure is less dependent on the amount of gas American drivers consume."
The study follows a Monday report from the American Public Transportation Association that said Americans turned to public transportation in the third quarter at their biggest jump in 25 years as gas prices hit record levels.
More than 2.8 billion trips were taken on public transportation nationwide from July to September, an increase of 6.5% compared with the same period last year, according to the APTA.
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