FOR ABOUT 31 months, there has been no federal law authorizing aid to surface transportation programs. Money has continued to flow under a series of temporary extension measures, which is not unprecedented — the last law was enacted 22 months after the previous one expired. But it is hardly the ideal way for Congress to do business. State governments, construction contractors and localities need the predictability that would come from a fresh five- or six-year bill, which congressional Republicans and Democrats and President Obama have promised — but no one can deliver.

The latest extension runs out June 30. The only vehicle that offers even short-term certainty is a two-year, $109 billion bill that passed the Senate 74 to 22 on March 14. The Republican House couldn’t produce even that much, and so it brings yet another 90-day extension bill to the conference committee negotiations that begin Tuesday.

Both parties would probably like to send a wad of road-building cash to their constituents during this election year, and the GOP has exploited that by trying to link a measure starting the Keystone XL pipeline to the transportation bill. But Democrats are resisting. The battle over Keystone has caused much delay and could end up making it impossible to get a bill at all this year.

Even without the Keystone issue, though, there would have been disagreements on transportation policy issues — such as how much to devote to mass transit and how to speed up often lengthy environmental and other reviews. Those issues cut across partisan lines, which is one reason Republican leaders could not muster a majority of their own House members behind a full-scale alternative to the Senate bill.

The deeper problem is what the parties agree on: No one wants to raise the gas tax, upon which the Federal Highway Trust Fund depends. Stuck at 18.4 cents per gallon since 1993, the tax has lost much of its value to inflation and is no longer sufficient to pay for the country’s transportation needs, especially in an era of more fuel-efficient cars and trucks. Consequently, the trust fund is nearly out of cash. The Congressional Budget Office says that the fund’s highway account will go to zero some time next year, and the mass transit account in 2014.

Instead of telling their constituents the truth — that well-maintained roads cost money, and some of that money has to come from motorists who use those roads — both Republicans and Democrats proposed to finance transportation through accounting gimmicks and stop-gaps. (An honorable exception was Sen. Mike Enzi (R-Wyo.), who at least floated the idea of indexing the gas tax to inflation.) The Senate bill, for example, would pay for part of its two years’ worth of spending with some $743 million that would be raised over the next 10 years. (The money would be raised by giving the government the authority to revoke the passports of tax delinquents, which analysts predict would encourage many of them to pay up.)

If passed, the Senate measure would consolidate overlapping programs and modestly streamline project approvals. It would buy time for Congress to consider a permanent financing fix, while construction projects proceed without fear of a sudden Trust Fund bankruptcy. If that sounds like faint praise for the bill, that’s because it’s the only kind it deserves.