prior probability

Alex Tabarrok posted the following yesterday on Marginal Revolution:

Here from Alain Bertaud and the Urbanization Project is another way of thinking not just about the high cost of free parking but also the opportunity cost of streets. In New York City, a place with some of the most valuable real estate in the world, 26.6% of the land is devoted to unpriced streets (and an even larger percentage once we include parking). In Manhattan we go to great expense and effort to make it possible for hundreds of people to use the same 10*10 square feet of land, we build skyscrapers, and yet at the same time similar quantities of land are being taken up by a few people and their cars.

 Hat tip: Brandon Fuller.

– See more at: http://marginalrevolution.com/marginalrevolution/2013/07/the-opportunity-cost-of-streets.html#sthash.3t9mw4yd.dpuf

My question, however, is this:

Since streets are a public good, should we expect  governments to provide too many streets or too few?

This is prior probability‘s second blog post … we wanted our first post to be something visual because lawyers and law professors often tend to express our ideas solely in words, but sometimes, ideas can be communicated more directly and more powerfully through visual images. For example, prior probability is currently writing a paper in which we try to visualize the problem of “probabilistic proof” in law. See http://papers.ssrn.com/sol3/papers.cfm?abstract_id=2271870

So what idea or ideas does the picture below convey?