Lawmakers have been grappling with ways to cover a gap between the amount of revenue brought in by the federal gas tax, which is traditionally used to fund transportation projects, and the amount of spending in each chamber's proposal for a new highway bill. The gas tax generates approximately $36 billion a year for the highway trust fund, but the House and Senate's original proposals each called for spending more than $50 billion per year on transportation.
The Senate proposed a package of tax loopholes and fee increases it said would generate most of the difference, but the group of lawmakers lobbying to protect the land and water conservation funds said the upper chamber opted to steer clear of the trust fund.
The House originally proposed paying for the difference with increased domestic oil drilling, though the lower chamber's proposal for a five-year, $260 billion measure struggled to win support from conservative Republicans in the chamber.
The highway conference committee has until June 30 to reach an agreement on a transportation bill before the scheduled expiration of the current mechanism for funding road and transit projects. The government's authorization to collect the gas tax is also included in the transportation bill.