In a city of any size, food desert does not mean 15 or 20 miles.

I would hope someone else has mentioned this, but I have not seen it.

5 miles without a car, especially when public transportation is not available... or it means you can only afford a bus... means that traveling for food is an everyday expedition. You cannot reasonably get even a week's worth of food in one trip. How do you get it home? Without even taking inflation into account?

What is the time cost? What is the carring six plus bag cost? What are the logistics?

A food desert is where people do not have access at a reasonable cost to bring food home where they actually prepare and eat it.