The straight dope:
"Packard Jennings and Steve Lambert asked architects, city planners, and transportation engineers, "what would you do if you didn’t have to worry about budgets, beauracracy, politics, or physics?" Ideas from these conversations were then merged, developed, and perhaps mildly exaggerated by Steve and Packard to create a series of 6 posters for the San Francisco Arts Commission’s Art on Market Street Program."
Have a look for yourself; the Zipline one you see to the left is an excerpt from one of them. Enjoy them all!