Tuesday, June 25, 2013

Citi Bike at Age 4 (weeks): By the Numbers

Citi Bike, New York's much anticipated, sometimes feared, and greatly needed bike share system, has now been in operation for four weeks. I have all manner of qualitative observations from this first month of service, but for now I'm going to lay out some quantitative analysis of the system's debut and early adoption.

I'm working with data published daily on the Citi Bike blog, including some useful figures (daily trips, new membership sales) and some of questionable value (daily miles, average trip duration, most popular stations). That latter category is not totally useless: over the weekend, the system reached the fun milestone of 1 million miles traveled, which makes for good press; and I suppose you could calculate the average systemwide speed, [EDIT: I did this just for kicks, and it's a mess. See below.] but it's the former on which I'll focus in an effort to track Citi Bike adoption and penetration rates.

Let's begin with the raw data. Every day, the blog provides sum totals of annual memberships sold, miles traveled, and trips taken. Antonio D'souza has been graphing these numbers:

In fact, it was Antonio's graph that inspired me to dig a little deeper into the Citi Bike numbers. Graphing the total figures for each day is good if you just want to show that the system is getting use, but it's not particularly revealing. Since these are totals since inception, they will of course only ever increase. Instead, we want to examine daily differences - the first derivative of the total numbers - to see whether the rate of growth is increasing.

Some daily increments are given on the blog (such as daily trips taken), and it's easy to derive the rest by subtracting, for example, total annual members on June 22 from total annual members on June 23. Here, then, is a chart of new membership sales and trips taken per day:
As you can see, this is quite a noisy graph. The fact that there is so much daily variation makes it difficult to discern longer-term trends. For instance, was the system somehow four times better or more appealing on 6/9 than 6/7? Certainly not; 6/7 was very rainy, and 6/9 was a beautiful Sunday. We will examine the weather/trips relationship in a future post, but let's now deal with the weekend effect - the spike in ridership that occurs, unsurprisingly, on Saturdays and Sundays.

(It's interesting to note in passing that daily ridership initially tracks quite closely with daily 24-hour passes sold, but has begun to diverge in the past week. This new trend, along with the recent strong weekday ridership, could be an indication that annual members are making more extensive use of the system - in particular for utility trips during the work week. Enabling such utility trips is a key goal of the system.)

To neutralize the weekly periodic fluctuation in Citi Bike data, we will calculate a rolling average over 7 days around each data point. This way, each number will reflect the average value over each day of the week. Financial applications usually use a trailing average, in which each point takes the average of the preceding 7 days, while scientific analysis generally employs a centered average, taking the mean of the week centered on the date in question. Here's what each of those options looks like with our data:
This chart makes clear the advantage of the centered rolling average: as Wikipedia notes, it "ensures that variations in the mean are aligned with the variations in the data rather than being shifted in time." (The trailing average is three days behind the centered average, which results in consistent underestimation when the data are generally increasing, as is happily the case with Citi Bike usage.) The chart also demonstrates how the rolling average neutralizes the periodic peaks and troughs on weekends and weekdays.

(By the way, ridership has been substantially greater on weekends than weekdays. Here are the average number of trips per day of the week:
  1. Sunday: 23,615
  2. Saturday: 18,196
  3. Thursday: 17,030
  4. Friday: 14,359
  5. Wednesday: 14,214
  6. Tuesday: 12,729
  7. Monday: 11,480
Monday is probably dragged down somewhat by opening day, which had only 6,000 trips, and Friday by that very rainy day on 6/7; I'm not sure why Thursday is so much more popular than the rest of the work week!)

Now that we have identified the data of interest (measures of change over time) and smoothed them to eliminate periodic noise, we can get down to assessing the success of Citi Bike's rollout in more detail. Let's look first at sales trends: how many new annual, weekly, and 24-hour memberships are being sold? Ideally, we'd want to see the 24-hour and weekly numbers steadily increasing, and the annual numbers holding steady or falling only slowly. (It's unreasonable to expect that annual sales won't drop off as the buzz from the launch subsides and the early adopters complete their early adoption.)
The data suggest that Citi Bike is performing quite well as measured by sales. Daily sales of 24-hour memberships have bounced around 2,100 for some time, but have recently skyrocketed; it remains to be seen whether that trend will hold up going forward. Weekly memberships initially failed to gain traction, dwindling to only 170 sales on 6/10, but have since gained traction, hovering around 400 daily sales since 6/19. Sales of annual memberships did see a fairly rapid, steady fall after launch, but stabilized around 6/10 and remained encouragingly flat through 6/20, since when they appear to have begun another slow decline. Even so, many hundreds of New Yorkers continue to join Citi Bike daily, nearly a month after the system's launch. This is heartening.

What about ridership? We know that people are buying memberships, but are they actually getting out and riding the big blue bikes? Let's compare the rolling average of daily trips to the number of active members (all annual members + new 24-hour members + 7-day passes purchased within the past week) on any given day. This figure - trips per active member - will serve as a rough proxy for Citi Bike's total mode share. (Rough, because 1 trip on Citi Bike does not necessarily imply 1 fewer trip on other modes, especially when the Citi Bike trip is recreational in nature. For instance, I recently took two trips on public transit in order to make several Citi Bike trips; but that's another story.)
This is perhaps the most heartening chart yet. As the total number of active members climbs steadily, the number of daily trips has dramatically risen (from about 12,500 on 6/10 to nearly 30,000 on 6/23), with a concomitant surge in trips per active member from a low of 0.32 to a recent high of 0.57. We may hope to see the trips-per-active-member figure continue to rise (surmounting 1.0 would be a great milestone), but the data so far suggest that Citi Bike is already becoming more and more important in the daily lives of its members.

What's next in Citi Bike data analytics? The interactive charts in this post will continue to update as more data becomes available over time, but of course my commentary will not automatically adapt to reflect new trends. I'll try to check in every so often and address recent developments as Citi Bike continues to expand its presence in NYC. You can always visit my Google Spreadsheet to see the latest numbers.

Of course, I'm not the only one exploring Citi Bike data. Greg is doing some great work analyzing the live station activity feed, and he's identified the most popular neighborhoods and come up with some good insights on best balancing practices. OpenPlans and betaNYC are holding a Citi Bike data night tomorrow. And there's a lot more data on the way, if NYC Bike Share makes good on its promise to follow Capital Bikeshare in releasing a web dashboard (though I hope they don't bury their data in a Silverlight application).

EDIT: For fun, I calculated the daily average systemwide speed (based on the otherwise useless miles per day and average trip duration data points). Here's the graph:
This is troubling, because the values are so implausible. During the first week, the claimed average speed (on those bulky bikes) is around what I attain when I'm riding my road bike through the city in a hurry (I do stop for red lights, though) - that is, it's unrealistically high. After that, though, it gets even worse, as the speed is almost always given as 7.46 +/- 0.02. I find it very difficult to believe that Citi Bike users just happen to achieve almost exactly the same average speed day after day (albeit a speed that sounds realistic given the bikes and their riders).

I'm not sure what to make of this suspicious data. It certainly shows the need for independent validation of blog data (using, say, the JSON feed). I'll keep an eye on this, though, and maybe inquire about it at tomorrow's bike share data evening.

Sunday, June 23, 2013

Too many bikes!

(Bikes, or yikes?)
You may have read the article about Amsterdam's bike parking problem in Friday's New York Times. Here, from New Amsterdam, is my take on the situation.

I've thought a lot about the end game for cycling as a mode in cities. Obviously bikes are much better than cars in terms of their environmental impact - pollution, noise, energy use, taking up lots of space. But one key problem - the domination of public space for the storage of private vehicles - remains an issue with bikes.

One piece of good news is that the day of reckoning is still way far off, at least for NYC. According to the article, 32% of trips in Amsterdam are by bike, a truly staggering mode share. (The current goal of the NYC Department of Transportation, the most bike-friendly DoT in city history, is for the cycling commuter mode share to hit 3% by 2020.)

I think there are two ultimate solutions to the problem of transportation in very dense environments. The first is a reliance on mass, rapid transit (i.e. subways and regional/commuter rail) for at least the backbone of most trips. Mass transit, as the name might suggest, has throughput rates that no other mode can even begin to approach. And if it's below grade (underground) or even above grade (but that's ugly; we took down the elevated railroads for a reason) it doesn't place heavy demands on street space, leaving that resource open for…

…paratransit - bike share and taxi fleets. One of the worst problems with cars is that they spend 97% of their time parked, rather than being in use. When that parking occurs in a dense urban setting, it represents a massive misuse of valuable space. (Why does Manhattan still have free on-street parking when adjacent condos are selling for $3,000 per square foot, and San Francisco realtors are selling garage spaces for upwards of $80,000?) Taxis and bike share are good because the vehicles are always in use or available for use, meaning the space they occupy is better utilized.

So ideally, trips in a central city would start on foot or via shared bike/car, connect to a high-throughput mass transit network, and end with a "last-mile" leg again on foot or by paratransit. (Or the shared bike or car trip could go the full distance, though that's somewhat less system-optimal, particularly in the case of the car due to the congestion it creates.) Outside the city? I care not; space is at much less a premium.

Denmark, as I understand it, actually works in much this way; many suburban Copenhageners own cars and drive them, for fun, on weekends, outside the city. Then they get back to transit and biking, much space-friendlier transportation modes, for their weekday utility trips.

So what should Amsterdam be doing to solve their (I can't believe I'm writing this) bike congestion problem? Building more bike parking is not a long-term viable solution, unless there's a sense that the 32% mode share is near the theoretical maximum (and I don't think it is) so parking needs won't continue to increase in future. Instead, they need to address the inefficiencies present in their city as indicated by the guy quoted in the article as having three bikes all locked up on public streets at the same time.

This misuse of public space has two good solutions: encourage a transition to bike share and begin charging for on-street bike parking. The Netherlands has a national bike sharing program, but it's smaller than NYC's system even as currently implemented (theirs totals 6000 bikes at 230 stations; we so far have about 5000 bikes at 300 stations and we're looking at 10,000 bikes and 600 stations hopefully by the end of the summer). A heavy expansion in central Amsterdam - we're talking a station at every other intersection - combined with an education campaign touting the benefits of bike share over private bike use (and there are many, not least of which is the nonexistent risk of theft) could get many Amsterdammers' private bikes off the road.

The bigger solution, and doubtless the more controversial in a city actually dominated by an "all-powerful bike lobby," is to charge bike riders for parking their bikes on public streets. This could be done through a system of handlebar-mounted permits (monthly or yearly for local residents or frequent commuters) allowing owners to lock up anywhere in the permit's district, and the installation of combined bike racks/parking meters for short-term use (ideally with an SFpark-style smart pricing infrastructure, so that prices can adjust over time to reflect local supply and demand). I envision the parking pricing remaining fairly low: the goal is not to raise money for the city, or to disincentivize biking, but rather to urge riders to store their bikes off the street - which would in turn invigorate private bike parking operations.

The bottom line is that in Amsterdam, cycling has become the new normal - which, thankfully, is the current goal here in New York and in an increasing number of cities. Cycling is an excellent urban transportation mode. But when it reaches the point where it begins to create the same problems that private car use has been creating for the past century, some of the same solutions are in order.

Does this suck? A little bit. One of the joys of the bicycle is the air of freedom it carries - the ability to zip nimbly through narrow streets, lock up to just anything, and get on with your business before the car driver even makes it past the choked expressway. I recently, after a debate with a fellow cycling advocate, made the decision to start riding not just according to the spirit of the law (don't endanger others) but also to its letter (stop for the full duration of red lights, at stop signs, etc.). I feel good about being part of the new, courteous face of urban cycling, but a little magic has gone out of the world for me.

In the end, though, it's not about the joy of riding (there's always suburban and rural roads for that sort of thing) but about the proper place of cycling in cities: safe, respected, ubiquitous - and regulated.

Where do you see cycling headed in NYC and Amsterdam?

Monday, June 17, 2013

New blogging home - and reasons why

I was just getting started using WordPress.com when I came up against a hard technological limit: WP.com doesn't allow JavaScript embedding. (Traditional, self-hosted WordPress installations appear to have no such limitation, BTW, so they're still a good option.)

Embedded JavaScript is a key feature for me, as it allows me to incorporate full-featured Google Charts into my posts. This will be particularly crucial for my series on Citi Bike analytics.

Combine the no-JS rule with the fact that it costs $13 annually (!) to enable domain mapping, and you have a recipe for my switching to Blogger, which allows both features, for free.

My first few posts are still online here. I may migrate them over to this page at some point in the future.