Any “Plan B” for enhancing transportation in congested metro Atlanta is bound to focus on what should be built, especially after last year’s rejection of the T-SPLOST.

That’s understandable. Much of the T-SPLOST debate concerned the merits of the specific road and mass-transit projects the 10-year, $7.2 billion sales tax would have paid for.

But deciding how to pay for new projects is just as crucial.

A sales tax, a la the T-SPLOST, is not the ideal way to pay for transportation. Of all the things state and local governments do, paying for transportation infrastructure is one of the simplest to link to usage.

With the gas tax, toll roads and fares paid by transit riders, the majority of transportation funding in Georgia comes from a user fee. A sales tax moves us away from that and is bad policy, from the standpoint of both economic efficiency and fairness.

In any case, we don’t necessarily need new taxes to increase transportation funding. That’s one message from a comprehensive transportation plan for metro Atlanta unveiled this past week by the Reason Foundation.

There’s much to be said (mostly good) about the particular road and transit projects in the plan produced by Baruch Feigenbaum, a metro Atlanta resident and transportation analyst for both Reason and the Georgia Public Policy Foundation. Equally important is how he’d pay for them.

The American Petroleum Institute reports Georgia has the nation’s 20th-highest combined state and local taxes on motor fuel. But only about four-fifths of the state motor fuel tax — and none of the local portion — is dedicated to transportation funding.

So our 20th-highest gas tax translates to per capita transportation spending that puts us in the bottom five states.

From a state perspective, between $150 million and $180 million per year in gas tax goes to the general fund for the Legislature to appropriate, rather than to transportation projects. That’s not as much as the T-SPLOST would have raised in metro Atlanta alone, but it would be a good start toward boosting money for roads and transit.

Normally, taking that money out of the general fund would mean cutting other spending in the budget. But the state projects a surplus of $600 million for the 2014-15 budget year. Legislators could rip off the bandage, so to speak, and shift the entire amount at once. Or they could make a more gradual shift. Or they could choose a lesser option, and simply allocate the money to transportation on an annual basis, so as not to handcuff themselves in leaner years.

Each scenario would provide more money for badly needed transportation infrastructure. Shifting the additional gas tax to transportation permanently would have the added benefit of constricting the amount legislators could spend on other, less vital programs and pork-barrel projects.

Feigenbaum also suggests something else the T-SPLOST ignored, to its great discredit: the possibility of using public-private partnerships to get more bang for the taxpayer buck. While his 30-year plan comes out to $28.1 billion, he estimates a quarter of that could come from private investors who then operate toll lanes or bus concessions and keep the fees paid by users.

Whatever one thinks of Feigenbaum’s project list, legislators would be wise to start moving in the direction he advises to pay for what we need.