Fly Cab uses the open-source programming language/environment Processing to render the path of a single San Francisco cab over a period of 5 days. The cab is represented by a yellow dot, and as it moves, it leaves a white trail behind to show where it's been. The dot moves 2000 times as fast as the cab it's representing, so the five days pass in a little less than 4 minutes.
As soon as a point on the cab's trail is drawn, it starts to sink down, so that each new point on the trail is slightly higher than the point preceeding it. As the trail gets longer, it builds a 3-dimensional structure of the cab's travels through the city over time. By examining the structure you can see not only what parts of the city the cab has been visiting, but how often it vists them.
As you're watching the cab travel, you may notice occasional stutters, abrupt jumps from one part of the city to another, or long spikes that can't possibly represent a real vehicle's path. These anomalies are a different sort of map--one of the reliability of the cab's GPS receiver, whose once-a-minute reports may be blocked by the city's geographic features, scrambled by hardware malfunctions or just lost in transit.
Thanks to Michal Migurski who suggested taking the path into the third dimension.
Tomas Apodaca is an interaction designer for Stamen Design.