FWIW The data for Milwaukee appears incorrect - the traffic fatalities are for the county of Milwaukee while the density is for the city. Recalculating density based on the land area of Milwaukee county shows 3,934 people per square mile.
I imagine it is quite close, the county of Milwaukee is essentially just the city. Even many of the suburbs are in surrounding counties
EliottM is right, the delta in land and population between the city and county is significant. Over 900K people in the county and 241 square miles compared to 96 square miles in the city.