WHILE Americans wonder whether the government will keep paying its bills, Congress is also debating how to keep roads paved and trains moving. The nation's primary transport law, which sets guidelines for spending levels and priorities, expired in 2009. It has since been extended in a series of short-term measures, the latest of which runs until the end of September. Pressed by state governments for a long-term replacement, legislators are rolling out proposals. Short-term extensions create uncertainty about federal funding and make local transport planning difficult. Yet after two years of delay, the parties are no closer to agreement.

The stumbling block is, as ever, money. Revenues from excise taxes, primarily those on petrol and diesel, are the main source of funding for federal highway and transit spending. At 18.4 cents per gallon, America's petrol tax is famously low and hasn't gone up since 1993. With petrol costs rising, drivers are filling up less, so the tax take is falling (see chart). Transport funds have repeatedly dried up in recent years, requiring top-ups from Congress—$30 billion-worth since 2008. Any new transport law must balance the books, but neither party is keen to find new revenue.

Democrats want more investment in the nation's crumbling infrastructure. President Barack Obama's 2012 budget called for a near-doubling of transport funding, to $556 billion over six years. Unable or unwilling to identify new funding sources, Democrats are scaling back those plans. A bill proposed in the Senate merely maintains current spending levels. Constrained by the falling tax take, the bill would run for just two years, and it still leaves a $12 billion gap to be filled from a yet-to-be-identified source. Party leaders can only hope that by 2013 the current anti-tax mood—or the balance of power in Congress—will have changed.

The Republican solution to reduced funding is far simpler: spend less. John Mica, chairman of the transport committee in the House, is pushing a full six-year reauthorisation with no new funding source. Federal spending would fall by about 33% as a result. Mr Mica aims to bridge some of the gap by encouraging public-private partnerships in new investments, and by opening up the system to private competition where possible. Democrats call this bill a “road to ruin”.

Greater private involvement is a good idea. With the economy weak, however, businesses may be no more able than local governments to fill the gaps left by cuts in federal spending. And America's roads and railways are deteriorating; the Congressional Budget Office estimates that $20 billion more is needed each year just to keep them in their present poor state. If the parties are unable to bridge their differences, the gridlock in Washington will quickly find its way to streets around the country.