Detroiters get around in all kinds of makeshift ways. Many don’t own a car, but do live near vacant land that makes direct line travel easier. Take a route enough times and a footpath is made in the ground. Repeat it even more and the groove will be so deep that it’s visible from space.

Joshua Newell, an assistant professor in the School for Environment and Sustainability at the University of Michigan, along with a colleague from Illinois State University, studied these informal footpaths, also known as “desire lines,” in a paper recently published in the journal Landscape and Urban Planning.

In what the authors are calling the first ever study of its kind, they found that these footpaths matter a great deal to the people who use them. And also that they’re rapidly disappearing.

To gather the data, they mapped desire lines across the entire city using Google Earth, verified them using Google Street View, and physically visited 46 city parcels. They also did in-person interviews with residents in the lower east side.

In total, they identified 5,680 informal footpaths, which adds up to 157 linear miles—approximately the distance from Washington, D.C. to Philadelphia. Around 25 percent got “heavy” use, which means people walked along it so frequently that the ground was bare.

“We were shocked to find out how many there are,” Newell says.

Through interviews they found that these routes most frequently act as a shortcut, but there’s other, more ambiguous reasons desire lines emerge. People might also enjoy a greener path to their destination or use it as a means to assert control over a place. “They can be expressions of people’s desire to use quasi-public land in the way they want, and have some say for how it should be used,” Newell says.

In other words, they’re demonstrations of Detroiters’ innate desire for getting around Detroit.

But, the study says, “Detroit is rapidly losing its desire lines.” In the lower east side, 70 percent of these footpaths disappeared between 2010 and 2016.

Some of this is attributable to development. Over 100 footpaths were eliminated because of Hantz Woodlands, a huge urban tree farm on the east side. Others were lost because of fencing, mowing (as well as lack of mowing), and parking.

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What, exactly, is lost when an informal footpath disappears is hard to say. Increasing the walkable distance between two points may make it less likely someone will make the trek.

But Newell thinks desire lines can be one part of a redevelopment strategy, especially in areas where there’s lots of vacant land. Not everything needs to be built; the city could encourage green space, stormwater management, and also formal footpaths. These land uses could contribute to walkability in the city while also improving aesthetics and decreasing crime, which was a concern among parents who participated in the study.

“Heavily used lines as connectors could be considered part of a solution, a piece of the puzzle,” Newell says.

Informal footpaths are clearly an important means of travel for Detroiters. Whether they’re worth preserving, and how to do so, are interesting questions.