Setting aside race, to me it seems like "economically disadvantaged populations share a disproportionate burden of traffic-related air pollution exposure" makes this pretty clear. <shrug>
Even into the 1960s wealthy black people were prevented from buying and living in the 'good' parts of even northern US cities.
[1]: https://www.theguardian.com/us-news/2019/apr/04/new-york-sou...
Density doesn't equal exposure.
I suppose that there likely are small numbers of people living far outside of cities and away from traffic, like up in the mountains of Appalachia or living off the grid in tents or in communes deep in forests, who are poorer than people you'd typically find in urban areas, but it's still the poor in the US who are dealing with the worst of the harms traffic causes compared to the better off/wealthy. They're also the least able to mitigate those harms or deal with the fallout caused by them.