  Here we show that "exposure segregation" - the degree to which individuals of
one group are exposed to individuals of another in day-to-day mobility - is
dependent on the structure of cities, and the importance of downtowns in
particular. Recent work uses aggregated data to claim that the location of
amenities can inhibit or facilitate interactions between groups: if a city is
residentially segregated, as many American cities are, then amenities between
segregated communities should encourage them to mix. We show that the
relationship between "bridging" amenities and socio-economic mixing breaks down
when we examine the amenities themselves, rather than the urban aggregates. For
example, restaurants with locations that suggest low expected mixing do not,
much of the time, have low mixing: there is only a weak correlation between
bridging and mixing at the level of the restaurant, despite a strong
correlation at the level of the supermarket. This is because downtowns - and
the bundle of amenities that define them - tend not to be situated in bridge
areas but play an important role in drawing diverse groups together.
