An unprecedented cutback in driving is slashing the funds available to rebuild the nation's aging highway system and expand mass-transit options, underscoring the economic impact of high gasoline prices. The resulting financial strain is touching off a political battle over government priorities in a new era of expensive oil.

A report to be released Monday by the Transportation Department shows that over the past seven months, Americans have reduced their driving by more than 40 billion miles. Because of high gasoline prices,...