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Traffic crawls in the eastbound lanes of Interstate 26 near West Montague Avenue in North Charleston on in May. A pre-filed bill in the S.C. Senate would shift some road maintenance funds toward interstate widening projects.

In 2017, South Carolina legislators passed a 12-cent state gas tax hike with the explicit understanding that the new revenue would be used to repair crumbling roads and bridges. It was a necessary step.

And so far, the results seem promising. In the first year, the state Department of Transportation made more than 180 miles of rural safety upgrades, repaved more than 2,000 miles of roads and repaired at least 50 bridges.

But although the full hike still hasn’t been phased in — the law capped the increase at 2 cents per year — related funds already appear to be a target for less crucial endeavors.

As reported by The Nerve’s Rick Brundrett in a column published this month in the Summerville Journal-Scene, a bill prefiled by Sen. Nikki Setzler, D-Lexington, would divert money set aside for highway maintenance to a new fund for interstate lane expansion.

In addition to raising the gas tax, the 2017 S.C. Infrastructure and Economic Development Reform Act tweaked the rates and allowable uses for some other transportation-related fees.

Mr. Setzler’s proposal would shift a portion of that revenue into a new Interstate Lane Expansion Fund.

While the bulk of the funding in question would still go toward maintenance, the proposed changes would nevertheless mark a significant departure from the gas tax bill’s stated purpose, which is to fix roads, not build new ones.

The broader idea — raising money for interstates — is not a new or inherently bad one. Mr. Setzler has pushed for an Interstate Lane Expansion Fund since at least 2014.

“Our interstates are crumbling and falling apart. They are in terrible condition. They are the backbone of economic development and can’t be forgotten,” he told the Associated Press that year.

Here, Mr. Setzler hits the nail on the head. But the proposed solution — adding more lanes — would actually exacerbate the problem of crumbling roads by both diverting funds that could be used for repairs and adding more miles of asphalt to maintain in the future.

It’s worth assessing the extent to which widening interstates could benefit South Carolina’s economy. The vast majority of South Carolina’s more than 850 miles of interstate are two lanes in each direction. As the state’s population and its popularity as a tourism destination grow, our transportation system needs to keep up with demand.

But wider interstates would need to be justified on their own merits and weighed against potential alternatives. They must be funded with the clear understanding that they are new miles of asphalt with new long-term costs.

That’s not what maintenance-related funds should be used for.

And given the sorry state of so many miles of existing interstate in South Carolina, repairs ought to take the top priority.