State leaders may have a difficult time planning long-term transportation projects, now that House Republicans have voted to make the Highway Trust Fund subject to the annual appropriations process, state transportation advocates said.

The fund pays for about half the budget of highway capital projects in most states.

Since 1998, all the revenue paid into the Highway Trust Fund, which provides about $41 billion in annual funding, has been used for highway and transit improvements each year; before that, Congress could reduce the trust fund spending when it wanted to increase spending elsewhere.

The new rule adopted by Republicans would once again make the fund subject to the annual appropriations process instead of bound by multi-year spending bills, according to groups such as the American Association of State Highway and Transportation Officials, which opposed the rule change.

The danger in that approach, transportation advocates say, is that unstable funding would make it difficult for states to make long-term plans.

Republicans have said the intent of their rule change was to prevent spending above allocated levels, but opponents of the change say the rule change has unclear language with loopholes that could affect spending.

Jo Maney, a spokesman for the House Rules Committee, did not respond to a request from Governing Thursday for clarification on the rule.

The rule change has drawn the ire of engineers, state transportation officials and builders, who say it will cause further uncertainty at a time when transportation infrastructure is already in desperate need of an upgrade.

"Long-term transportation plans and projects require stable, predictable funding," said Pete Ruane, president of the American Road & Transportation Builders Association, in a statement. "With multi-year reauthorization of the federal highway and transit programs now more than 15 months overdue, this sends the wrong signal to the states and Wall Street."

Critics of the move say it would allow Congress to reinstate gimmicks that permitted it to reduce spending from the fund, allowing it to build up a surplus and thus free up spending for other purposes.

"There are two deficits facing the country today — the federal debt and the deficit in maintaining the infrastructure on which economic recovery depends," said John Horsley, executive director of the American Association of State Highway and Transportation Officials, in a statement. "In their zeal to address the first issue, the new House leadership has taken action that deepens the second."