Population density & car ownership

The diagram below shows the relationship between population density and car ownership in the municipalities of the canton Aargau in 2010. There is a negative correlation between both factors, what means that higher population density leads to lower car ownership and vice versa.

A likely explanation for this result is that high densely populated areas have high concentration of travel demand. This concentration may imply a higher risk of traffic jams and better conditions for high frequency public transport services, what may contribute to reduce car use or car ownership.