SACRAMENTO — As state legislatures across the nation grapple with declining funding to maintain roads, bridges and other transportation infrastructure, California lawmakers are confronting the unintended consequences of policies aimed at achieving ambitious environmental goals.

Gov. Jerry Brown has called on the state to expand its landmark global warming law by slashing gasoline consumption in half and adding 1.5 million zero-emission vehicles by 2030.

But California depends on taxes paid at the pump to fix roads. For more than 20 years, the state and federal government have charged separate 18-cent per gallon taxes for road maintenance, a rate that has not kept pace with inflation or been adjusted to address falling fuel use as Californians drive more fuel-efficient vehicles. State revenues slid from $2.87 billion in 2003 to $2.62 billion in 2013, even as the state's population has increased and vehicles are driving more miles.

"Each year, we fall further and further behind, and we must do something about it," Brown said in his January inaugural address.

Federal gas tax money is also drying up.

Figures compiled by The Associated Press show the total amount of road money available to states from the Federal Highway Trust Fund has declined 3.5 percent during the five-year period ending in 2013, the latest year for which numbers were available. During that span, the amount of inflation-adjusted federal highway money dropped in all states except Alaska and New York.

California saw federal funding plunge 13.5 percent to $3.8 billion in 2013.

Brown has called on lawmakers to address a $59 billion backlog in state infrastructure repairs over the next decade.

In response, Assembly Speaker Toni Atkins, D-San Diego, has proposed an annual $52-a-year fee for drivers in addition to gas taxes.

Atkins' plan is in its early stages and her office has provided scant details, but she says motorists could be charged along with their annual registration fees or through their insurance at different rates depending on what vehicles they drive. She says potholes and cracked roads already take a toll on Californians' wallets through repairs to damaged tires and shocks.

Lawmakers in other states facing similar quandaries are proposing gas tax increases or flat fees for electric car drivers. In California, raising such fees would require two-thirds support from the Legislature, including from tax-averse Republicans.

GOP leaders in the Legislature say the state should look for infrastructure repair money elsewhere in the budget before asking Californians to pay more. They have suggested redirecting $1 billion in truck weight fees being used to pay off state debt. They have also blasted the California Department of Transportation for overstaffing, citing a legislative analyst's report last year showing $500 million in unnecessary personnel costs.

Lawmakers last year authorized a study on whether to tax drivers by miles driven instead of gas guzzled, but such a proposal could take more than five years to implement.

A so-called road user charge is also being studied in Minnesota and Nevada and will debut this summer in Oregon with 5,000 volunteers paying 1.5 cents per mile driven in exchange for potential gas tax rebates.