In reply to Paul Udstrand.

Simple:
- A higher share of Minneapolis residents commute and shop within Minneapolis than commute to SLP (or any other individual suburb)
- A higher share of SLP residents commute or shop in Minneapolis than the other way around.
- Of those moving within the city, a good chunk get by without using freeways for most trips. Many don't even drive (~25% walk, bike, or bus to work, 31% of all trips according to UMN research). People in SLP drive on much more expensive infrastructure to places like West End for a higher percent of their trips than Minneapolis.
- Freeways through the central cities enabled people (and businesses) who would otherwise have lived in those cities to move out. http://www.jstor.org/stable/25098858 Point being, the economic activity freeways allow in the city by people coming in would likely occur without them anyway, most likely higher than our current reality (the study notes that without the freeways our population would be 8% higher, with presumably more businesses as well).

There are no moral judgments in any of those statements, just pointing out that freeways benefit the average suburban resident more than the average Minneapolis resident, and thus the highway subsidies that flow to building/maintaining those roads (along with county/etc roads) benefits them more per capita as well.

None of these points really matter to the central part of the debate: whether a denser city is (fixing for residents' income) more or less expensive to serve per capita.