Our understanding of food access seldom takes into account a key factor: mobility.

Food deserts, as they’re traditionally defined, assume a world without transportation. The United States Department of Agriculture measures them this way: A Census tract is considered a food desert if it meets a certain threshold of poverty, and if at least 500 people or one-third of the population reside more than a mile from a large grocery store.

This definition assumes that people only access supermarkets within a fixed – and arbitrary – radius of their home. And it sidesteps the reality that many people ride transit or cars to get there.

"By using a static map that looks at the distribution of homes and food stores, you get a rough cut," says Michael Widener, an assistant professor of geography at the University of Cincinnati. "You get a general idea of what spaces have worse access than other spaces. But ultimately, these are just models."

And they’re models that completely miss the picture of human mobility. This is just one of many criticisms of food-desert maps (another common one is that they dismiss healthy food sources like corner bodegas). But if we could more realistically capture where people shop and how they move through their day – accessing grocery stores close to work, for example – that could help us better identify who’s really at risk and what would help them.

Widener cites three factors that influence access to healthy food: spatial access, economic access, and social food norms. And those factors vary in degree from place to place, even person to person. Maybe you have easy spatial access to a supermarket.

"But you can buy cheap Oreos at a supermarket as easily as you can buy produce," Widener says. "It’s just a much more complicated issue that drawing a buffer around a zone and saying ‘these people don’t have access to food."

In research conducted in Cincinnati, published in the journal Health and Place, Widener and colleagues have tried to tease out a better understanding of that first factor, spatial access, by looking at food deserts through the lens of commuting data. What if, for example, a person who lives in a "food desert" commutes every day by car to a job near a full-service Kroger?

For reference, this is a map of Cincinnati with USDA-designated food deserts drawn in red boundaries. Supermarkets and large grocery stores (as well as the Findlay Market) are identified in blue stars:

"Using urban commuting data to calculate a spatiotemporal accessibility measure for food environment studies," M. Widener et. all in

Health & Place

The researchers then drew on 2005 data from the Metropolitan Planning Organization in the region to model commutes across 359 "transportation analysis zones" in Cincinnati for approximately 158,000 car commuters. Within the city, Census data suggests that 71 percent of commuters drive to work alone. The MPO data then sketches patterns of where those residents commute to work from home, enabling the researchers to then compare those destinations to current data on supermarket locations.

They then measured geographic access to food as a metric of time: If you’ve got two hours to spare grocery shopping on your way home from work, how far out of your way will have you have to go to get to the store (and how much time will you have remaining to spend there)?

In this map, the transportation analysis zones shaded the darkest green are those with the highest access to food (or, put another way, those where residents have the most time to spend shopping on their way home). Wherever these residents work – accounting for all of their work locations – this score from 1-120 minutes reflects the average time they’d likely be able to spend shopping. The higher the number, in other words, the less time they spend going out of their way to find food (the two hour mark is simply a baseline).

That previous map illustrates commuters visiting the grocery store on their way home from work. This map illustrates instead access to food for drivers departing from and returning to their homes (as they might do on a Saturday afternoon):

This last map represents a ratio of the previous two. Are there actually any zones where people have more access to food when leaving work than they would if they departed from home? Do they gain access to food as a product of their regular commutes?

In fact, there are two zones that fit that description (in yellow). And one of them is identified as a food desert by the USDA. Widener admits that this number isn’t terribly impressive. "But for the purposes of academics," he says, "it proves the point that there are cases where mobility does actually have a significant impact on how much access a person might have [to food]."

It’s important to note that this study only examined car travel. As a next step, the researchers plan to look at transit commuters (they make up 9 percent of Cincinnati workers). This is the population most likely impacted by poor access to healthy food. But this group will also be an even harder one to study. Even if you’re a commuter with great transit access to supermarkets, you probably don’t want to lug as much food onto a bus as you’d put in the trunk of a car.

Mobility, however, does color our existing picture of food deserts.

"It expands the range of possibilities," Widener says. "We can move outside of just ‘we need a grocery store in this neighborhood or that neighborhood.’ It would take a lot of economic incentives to drive a major chain to open a grocery store in an area they might deem as not being a profitable location."

But once you start thinking about mobility, other solutions seem feasible. Why not create express bus service from some neighborhoods straight to the supermarket?

Top image: Noel Powell/Shutterstock

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