IN A MORE rational universe, South Carolina’s gasoline tax would be a good bit higher than it is.

Just as it would be hard to justify a tax that is effectively among the highest in the nation, it’s hard to justify having one of the lowest taxes in the nation. And like our anemic cigarette tax, our gasoline tax is knuckle-scraping low. Just three other states charge less — which happens to mean that just three states do more to reward and encourage the gas-guzzlers that make our country dependent on Middle East despots.

Likewise, no one could rationally dismiss the argument that we need to be spending more repairing and improving our highways, whose inferior design and upkeep play a significant role in our top-10 highway death rate.

And supporters in the Senate have offered the tax hike as an alternative to a House-passed plan that steals money from the day-to-day operations of state agencies, the most crucial of which remain underfunded despite increases the Legislature approved for next year.

But our universe isn’t rational. And the policies our elected leaders have embraced make a tax increase for highway maintenance hard to justify.

As much as we need to improve our highways, that isn’t our most essential — or our most urgent — need. We have a greater need to make sure kids who attend our poorest schools have as much of an opportunity to learn as kids who attend our richest schools; to keep our prisons safe and to provide educational, rehabilitation and counseling services to the prisoners who are going to be released back into our communities eventually; to provide care for the mentally ill, rather than sloughing that off on the hospitals, which by law must treat them.

If those needs aren’t important enough to cut other services or else raise taxes to fund, then the highway maintenance problem isn’t either. You don’t raise taxes unless it is to cover your most essential unmet needs.

Raising taxes for road maintenance presents a particular problem, because the Transportation Department is one of the least accountable agencies in state government. The way it is structured, it is impossible for voters to hold anyone responsible for how it operates: One board member is appointed by the governor; the other six are appointed by the legislators who live in their congressional districts — and they can’t remove “their” board member until his term ends.

On top of that, serious allegations of cronyism, favoritism and misconduct have been raised about the administration and some board members; they must be dealt with before the agency receives any more money.

Finally, Senate leaders’ gas tax is just as unacceptable as the governor’s income tax cut and the House’s property tax cuts because they all treat an individual tax as an island unto itself. No tax is that. Rather, each one is part of a far-too-complicated, disjointed and layered-upon morass that constitutes South Carolina’s tax code. Change one, and you affect all the others — often in ways no one had anticipated.

What we need to do to meet our road needs is the same thing we need to do to meet our education needs and our public safety needs and all of our other needs: Figure out what services our government must provide, how much they cost and which taxes should be used to pay for them. Then we need to build a tax code that reflects those decisions. It’s not a quick and easy way of solving problems — but it’s the right way.