WASHINGTON — A Congressional Budget Office report warned this week that fuel economy rules proposed by the National Highway Traffic Safety Administration and Environmental Protection Agency could further reduce revenue for the already-stricken Highway Trust Fund, the primary source of financing for the nation’s surface transportation network.

The rule jointly proposed by the two agencies in 2011 would tighten corporate average fuel economy standards for new light-duty vehicles such as cars, sport utility vehicles, pickup trucks and minivans, beginning in 2017. That’s a problem for the Highway Trust Fund, which relies overwhelmingly on federal gas tax revenue. The CBO has previously projected that the fund is headed for insolvency in coming months without additional transfers from the U.S. general fund. The HTSA/EPA rule would cut revenue by 21% by 2040, the report concluded.

Despite the fund’s financial woes, exacerbated by a gas tax rate that has been stuck at 18.4 cents per gallon for nearly 20 years, states have still been willing to issue bonds backed by their expected shares of the trust fund. These bonds, known as grant anticipation revenue vehicles, or Garvees, have come under greater scrutiny in the market since Congress hit an impasse in passing new multi-year highway funding legislation. Rating agencies have warned that Garvees could be hurt by the increasing uncertainty of federal funding.

But even if the rule were adopted, the CBO report said, the combination of a phased enforcement — the strictest standards wouldn’t kick in until 2025 — and the slow replacement of older vehicles with newer ones means that the full effect wouldn’t make itself felt until 2040. Under the new rule, light-duty vehicles manufactured in the year 2025 or later would need to hit an average fuel economy of 49.6 miles per gallon.

Joung Lee, an associate director at the American Association of State Highway Transportation Officials, said the report continues the trend of pointing to a big future challenge.

“I think it is just the latest reminder about the current policy conflict between improving the energy footprint and paying for transportation infrastructure that will eventually have to be addressed,” he said.

The CBO already projected that the trust fund will be $147 billion in the red by 2022. If this proposal is enacted, that projection is $204 billion.

The report suggested that policy makers alleviate the problem by raising the gas tax, cutting highway spending, or transferring more general fund dollars to highways. All present major political hurdles.

The House and Senate are currently in a conference committee to negotiate new highway funding legislation, and aim to maintain spending at current levels.

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