Capacity and congestion is the wrong way to think about traffic planning. The natural response to capacity increases is to accept a longer-distance commute in exchange for lower housing costs. Instead, it's better to think in terms of accessibility - people need to go to work, buy groceries, etc, and the system must be able to accommodate those goals within reasonable amounts of time.

Crucially, this means that it's just as valid to ensure that people live within a fifteen minute walk to a grocery store as it is to ensure that traffic can allow people to drive five miles to the grocery store within fifteen minutes. Density itself can help traffic, so long as it is properly located with respect to jobs and various amenities.