Right, the overall density is a coarse measure of the resources available for infrastructure (presuming that they will at least correlate with population and distance). If you have higher density, it is very likely there will be relatively more resources to deploy in a given area.
It might be the case that density is not independent at all, and indeed is changed when you change the type of infrastructure.
(Not to mention that bicycle and pedestrian infrastructure is far, far cheaper than that for heavier vehicles.)