I watched a video of somebody rolling a marble down a particular sloped street in Manhattan. It took 3 minutes to get from one end to the other. Then the same thing, with a slightly-powered vehicle: it took 1-1/2 minutes. No great surprise. Anyway, I went to the street in question for some reason, saw a couple of suspicious-looking vans without license plates, concluded (somehow) that they were associated with the Bill Frist political campaign, whose office was right there, and climbed up the stairs to ask the campaign about it. The campaign office didn't appear to have a door, but only a window, so I had to climb around from the top of the stairs in through the window (hanging on by my fingers two stories above a busy Manhattan street). But I got inside without incident, and with some trepidation asked about the vans. The friendly-looking fellow sitting inside the cluttered office immediately guessed I was concerned about the lack of license plates and said "Oh, yes, those are 'adorned', a legal alternative to license plates" for some reason I don't remember. So I climbed back out the window, down the stairs, and took some photos of the vans just to make sure.