I love WalkScore a great website that lets you assess the walkability of various neighborhoods and features an ever-growing set of amenity data. But I think their new ranking of the Top Ten U.S. Car Sharing Cities is pretty misguided. Their approach seems to have been to do a crude count of how many car shares there are in any given city, leading to the conclusion that New York is number one, San Francisco is number two, Chicago is number three, Portland is number four, DC is number five, and so forth.
But this is apples and oranges. Chicago has 630 car shares but it also has 2.7 million residents and 227 square miles of land area. DC has about a quarter of Chicago's population and just 61 square miles of land to contain its 521 car shares. Portland has about as many car shares as DC (531) but fewer people and a much larger area. It's clear that either Portland or DC is a more "car share dense city" than Chicago, and then the question of comparing DC to Portland is how much do you weight population versus land area in assessing this kind of thing. In my experience as a Zipcar user, both the per capita quantity of vehicles (which determines scarcity) and the per square mile quantity of vehicles (which drives convenience) are important factors.
Meanwhile, common sense tells you that this is a question that's best assessed on the neighborhood level rather than the city level. The relevant issue is really whether the particular place you live is well-served by car sharing services, a question that's difficult to assess from municipality-level aggregates. That's the whole reason mapping-based tools like WalkScore are valuable in the first place!