DENVER (AP) As funding for highway construction and upkeep dwindles, Colorado has increasingly turned toward a once-unthinkable way to fund road expansion: tolls.

The federal highway trust fund gave the state nearly 9 percent less last year than in 2008 after adjusting for inflation, mirroring a nationwide dwindling of earmarked funds for road maintenance. But in Colorado, state budget woes and a unique tax limitation law have exacerbated the trend, according to transportation officials. The state’s Department of Transportation said it now spends $1.2 billion on roads when in 2007 it spent today’s equivalent of $1.7 billion.

Plans to replenish the agency’s coffers from Colorado’s newly flush general fund are being dashed by the state constitution’s requirement that excess revenues are refunded directly to taxpayers.

Nobody thinks the state can successfully raise its already-low gas tax. That has left government with two options, according to Don Hunt, the outgoing Department of Transportation executive director — improved cash management and tolls.

Reshuffling the way the agency manages its available cash has let it keep up with maintenance needs so far, Hunt said, but he called that “pulling the last rabbit out of the hat.’’

To expand lanes the agency is relying on private companies to charge tolls. Some residents last year objected to the state allowing a firm to charge vehicles for use of additional lanes on U.S. 36 between Denver and Boulder. But the state also plans to allow a company to charge tolls on new I-70 lanes both in the mountains and on the way to the airport.

“We just don’t have any other place to turn,’’ Hunt said. “We can’t grow by millions of people and not invest in transportation.’’

Democrats are increasingly talking about asking voters to let lawmakers keep some of the extra revenue generated by the recovery rather than refund it to taxpayers as required by the 1992 ballot measure known as TABOR, or the Taxpayers Bill of Rights. Transportation funding is one of the reasons why, they say.

“I think it’s very hard to figure out how you can add more general fund to transportation when we’re struggling still to adequately fund education,’’ said Assembly Speaker Dickie Lee Hullinghorst.

But Republicans say the state has to live within its means. GOP House Leader Brian Del Grosso contended that Democrats have not made transportation a priority and chose to spend elsewhere. “The average family has to live under fiscal restraints,’’ Del Grosso said. “We should not be any different.’’

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