Well of course where you live plays into what mode of transportation you'll choose, and the mode of transportation you'd prefer will often drive where you live.

Well of course where you live plays into what mode of transportation you'll choose, and the mode of transportation you'd prefer will often drive where you live. For example, I live a long walk from work (or a short bike trip, which is what I do 99% of the time; walking is mostly reserved for nice lazy evenings or blizzards). Yes, the rent is more expensive, but I'm also saving a lot of money on not having to buy/maintain/insure/park a car. And my daily commute is only around 4 miles round trip each day. So it does make sense to evaluate commuting mortality on a per trip basis, e.g. "what is the mortality associated with day of going to the office". The point of a trip isn't to go X miles, it's to go accomplish Y thing, such as going to work, running an errand, etc. Sparse suburbs shouldn't get "credit" and have their mortality normalized away on distance for everything being spread out. The sparseness itself is a problem in that it's requiring such long mileage for such trivial trips as "going to work" or "getting groceries".