Citi Bike Patterns: Tides and Rivers

It's a little after 7 p.m. now and the situation in lower Manhattan has changed a bit:

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As you can see, on net after the evening commute, Chinatown and the Lower East Side have gained bikes while the financial district has largely lost them (it's a bit mixed as you can see). Tribeca remains crowded with bikes. I assume this reflects commuting patterns, with Wall Street workers heading home and with Tribeca attracting an evening crowd for food and entertainment.

This is really the best case for the argument I made in my previous post for expanding the docks. Where stations are alternately full and empty, larger stations allow more people to use the bikes without adding much extra work for system employees. The analogy here is to a tide—the bikes come in, the bikes go out, all according to a diurnal cycle that naturally repeats itself. I'm not saying Citi Bike employees (and Citi Bike Angels!) never have to rebalance the bicycles, but in the ordinary course they just go back and forth with regularity. All you have to do to increase capacity is build more docks and add more bikes.

Stations that are perpetually full or perpetually empty are a harder case. In contrast to the tidal flow of lower Manhattan, the analogy here is to a river flowing from one station (or group of stations) to another. For instance in Park Slope most of the stations toward the bottom of the slope are in perpetual surplus, while the stations at the top of the slope (by the park) are in perpetual deficit. In effect there is a river of bicycles constantly going down the slope, presumably because people like to coast downhill but don't generally care to pedal uphill. Here's what it looks like (the slope is generally uphill as you go east):

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This is what Park Slope always looks like. I knew before I went to the map that I would find a suitable screenshot to illustrate my point. Individual stations vary, but the pattern of green stations at the bottom of the slope and yellow/red at the top is near-constant. (Occasionally the neighborhood gets inundated and everything goes green, but this never lasts long. By the way, that's one of the crappiest times to use the bicycles because there's nowhere to dock them. You end up going way out of your way and/or paying the fee for going over the time limit.)

If Citi Bike were to expand the stations in Park Slope, more bikes could fit in the lower part of the neighborhood, but within a day or two they would be full again and the ones at the top of the slope would be empty. It takes constant work to keep the stations in balance. It's not clear that increasing the number of bikes or docks would help much. The key problem is moving the bicycles each day. (With large enough stations maybe you could ensure that stations never run out of bicycles, but it would still take a lot of effort keeping them balanced.)

I don't have much of a point here, except to note that if Citi Bike were to expand its stations, it should expand them in "tide" areas more than in "river" areas. Station size is clearly the limiting factor in lower Manhattan, whereas in neighborhoods like Park Slope the key thing is simply to move the bikes uphill, against the current.