  The notion of the $x$-minute city is again popular in urban planning, but the
practical implications of developing walkable neighborhoods have not been
rigorously explored. What is the scale of the challenge that cities needing to
retrofit face? Where should new stores or amenities be located? For 500 cities
in the United States, we explored how many additional supermarkets would be
required to achieve various levels of $x$-minute access and where new stores
should be located so that this access is equally-distributed. Our method is
unique because it combines a novel measure of equality with a new model that
optimally locates amenities for inequality-minimizing community access. We
found that 25% of the studied cities could reach 15-minute access by adding
five or fewer stores, while only 10% of the cities could even achieve 5-minute
average access when using neighborhood centroids as potential sites; the cities
that could, on average, required more than 100 stores each. This work provides
a tool for cities to use evidenced-based planning to efficiently retrofit in
order to enable active transport, benefiting both the climate and their
residents' health. It also highlights the major challenge facing our cities due
to the existing and ongoing car-dependent urban design that renders these goals
unfeasible.
