There's something missing from the debate about Michigan's proposed 45-cent gas tax hike.

Gov. Gretchen Whitmer and the Michigan Department of Transportation are asking drivers to pay an additional $270 per year on average. In return, they say, Michigan drivers will get fewer potholes.

But that's only part of the story. Potholes are a big issue in Michigan to be sure — especially in some of the older cities like Detroit.

But MDOT isn't just asking for money for road maintenance. It's also for money to expand roads, to widen highways and state roads.

And whether the state really needs wider roads — new road capacity — is a question that deserves a serious discussion. Because widening highways is not only really expensive, it can cause problems as well, especially in areas like Detroit that don't have much population growth.

MDOT is currently moving ahead with two highway projects that include widening in the Detroit metro region: I-75 and I-94. The bill for those two projects is about $4 billion — or almost twice what Whitmer expects her gas tax to bring in annually.

It's hard to know exactly how much is for maintenance and how much is for widening. But University of Michigan Ph.D. student Joel Batterman estimates that between those projects and a list of other expansions being planned for Oakland and Macomb counties, about $1 billion of highway widenings are being planned for Southeast Michigan alone.

Keep in mind: Southeast Michigan — and Michigan itself — isn't growing. According to the Southeast Michigan Council of Governments, the region's population is about the same as it was in 1970. And is expected to remain fairly steady out to 2045.

Adding highway lanes in a region that isn't growing is not sustainable — and that's partly why Michigan has such a big pothole problem. It's hard to maintain an ever-increasing supply of highway lanes with the same population.

Highway widening projects also undermine urban areas and encourage sprawl by transferring land value from areas that are already developed to areas that are not. In other words, they help "new" places like northern Oakland county — who are the primary beneficiaries of wider highways — while hurting already developed places, like Detroit.

Whitmer and MDOT are probably right that there is a real need for more money for road maintenance in Michigan. But unless there is stricter focus on actually fixing roads, rather than building new ones, it could just worsen some of the region's most serious problems — further widening inequality and spurring more disinvestment in urban areas.

Angie Schmitt is the Cleveland-based editor of Streetsblog USA.

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