Saw this going round today:

http://www.flickr.com/photos/walking...750905/detail/

and for New York specifically:

http://www.flickr.com/photos/24431382@N03/6747484741

It's a very cool method, but I think New York throws it out with Broadway being the only diagonal in much of the city. Any shortest path algorithm will clearly favour it. But they're excellent maps of traffic flow in a city. It's almost worth stealing these and basing a totally new city map on the traffic flow patterns!