Someone was just telling me about this the other day.
German conceptual artist Wolf Vostell made a piece about this issue in Chicago as early as 1970, Concrete Traffic. A maquette for the piece is viewable in the exhibition The City Lost and Found: Capturing New York, Chicago and Los Angeles, 1960-1980, which I co-curated at the Art Institute of Chicago (on view through Jan. 11). The actual piece is under restoration now, as is detailed in these documents. It's pretty terrific and I had the pleasure of seeing one of his intact concrete cars in Cologne, Germany, recently. Part of the significance of Concrete Traffic was that by surrounding the car in concrete, he literally and metaphorically concretized the space that cars occupy since they were meant to be parked at a meter and the commissioning institutions had to pay for parking for the duration of the pieces. The fact that it could never be moved was an ironic way of pointing out all the space that cars occupy in our cities. That said, it was eventually moved and donated to the UofC where it sat for several decades.
Dag it, I was getting around to posting these, one from Latvia the other from Spain (?)
How about someone doing a kid's book of equivalents: how many bicycles could be made out of a single car? Bicycle tires versus car tires, aluminum cans into bicycles, etc. Stretching the reality of materials - but fun.
Once I asked Nancy Krum about how many vegetables could be raise in a parking space - she didn't think it would be viable, but imagine a big shopping mall, now filled with community gardens.