Jarrett Walker has provided a lot of insightful commentary and thought-provoking questions since he established his Human Transit blog a few months ago. His most recent series questioning the value of streetcars is in this vein: we should question every mode, because modes are just tools for accomplishing our goals.
In his most recent post and the previous one, Jarrett observes that streetcars don't improve mobility relative to buses. For him, mobility means "How many places can you get to in a fixed amount of time?"
Interestingly, Jarrett uses Walk Score to count the places, meaning that his mobility takes density into account. That makes it more valuable than simply measuring how many route-miles you have available to you. There was some back-and-forth in the comments about whether streetcars could increase mobility by increasing density relative to a similar investment in buses, but I don't think there was a solid conclusion.
My main response to Jarrett's post, though, is simply "Why?". He writes, "I'm just suggesting that the mobility offered by a transit service is an independently assessable feature that some people might want to decide if they care about." But do they care, and if they don't, why would they start? And why do we care whether they care? Because they'll start a campaign against this service? Because they'll stop using it and starve it of fares?
What I'm missing from these posts is a sense of overall goals. I've got them right up at the top of the page: "Reducing pollution, Increasing efficiency. Reducing carnage, Improving society, Transportation for all." I suppose that mobility would come under the headings of increasing efficiency and improving society. The five principles that I extracted from Transportation for America's torturous PDF were "Safety, Sustainability, Fairness, Independence, Prosperity," and I'd put mobility under Prosperity. For me, these are lower on the priority totem pole.
Of course, mobility can be used to sell a service, potentially drawing people out of their cars, and thereby contribute to more goals. But as with the anti-streetcar campaign and the loss of fare revenue, these are hypotheticals for a hypothetical streetcar. Why not cross those bridges when we come to them?
I think in order for us, Jarrett's audience, to value the goal of mobility, he needs to explain it in terms of our own goals - and not hypothetical ones. Otherwise it's a tool without an application. A provocative discussion, but not quite as productive as it could be.