Biking in NYC Is Up… But How Much?

The first time I counted bicycles in traffic was in 1988. There were rumors that Mayor Ed Koch was going to reinstate the Midtown bike ban that a judge had set aside the previous summer. As president of Transportation Alternatives, I dispatched a team of volunteers to Midtown avenues with clipboards and stopwatches. Our finding that bikes accounted for 8-10 percent of vehicles in Midtown traffic was independently confirmed by the city DOT commissioner. With cycling’s presence statistically validated, talk of the bike ban faded.

Several years later, as I was helping finish up TA’s Bicycle Blueprint, I gave myself a much harder task: to estimate the number of cycle trips in all of New York City over the course of a year. "We are traffic" was the new mantra, and it was time to take bicycle quantification to the next level.

The lone available datum, though, was DOT’s "screenline" count of 10,930 bikes crossing into or out of Manhattan south of 50th Street on a single day in 1990. I held my breath, made a bunch of heroic assumptions (e.g., how many Central Business District cycle trips never cross the screenline for each one that does, how cycling’s share of each borough’s total traffic compares to the CBD), and built an algorithm that extrapolated the screenline figure to the entire city.

My estimate: In 1990, 75,000 New Yorkers took a total of 265,000 cycle trips each day. Despite the guesswork, even a rough estimate was preferable to leaving the screenline figure to represent cycling citywide.

Fast forward to this year, when TA asked if I could update my estimates. I dusted off the algorithm and plugged in DOT’s latest screenline count, taken in 2009. The apparent finding was that on an average day last year, 236,000 people rode bikes for 1.8 million miles. Last month, though, when the Times’ David Goodman asked me to walk him through my calculations, I spotted a glitch: the city’s bicycle infrastructure and demographics had changed — in particular, the CBD’s share of overall city cycling had almost certainly fallen — but I hadn’t updated the ratios by which I extrapolated from trips crossing the screenline to trips within the CBD and in the boroughs. Realizing this, I backed away from my numbers, which contributed to the budding controversy here on Streetsblog and in the Times.

To firm up my numbers, I devoted much of last week to expanding and tightening my model.

To more precisely account for the dropoff in cycling in the fall and winter, for example, I drew on estimates of seasonal variations from Charlie McCorkell, the engineer and bike store impresario who has been observing cycling trends here since the 1970s. Charlie also helped me quantify weekend cycling, which bumps up the averages (as does nighttime cycling, which the DOT also doesn’t count).

Most important, I downgraded my ancient estimate of the ratio of cycling inside the CBD to screenline-crossing cycling, and I set Brooklyn’s bicycle mode share closer to Manhattan’s. (Figure 3 in Rutgers Professor John Pucher’s impressive new paper on NYC cycling [PDF] was also helpful on that score.)

The revised model produces these numbers:

  • On an average day in 2009, an estimated 201,000 different people rode bicycles in New York City. Their trips covered almost 1.7 million miles.
  • Estimated bicycle miles traveled are now (in 2009) 2.5 percent as great as motor vehicle miles traveled in the five boroughs. Based on that estimate, and averaged across the city’s streets, highways and greenways, 1 out of 41 vehicles in motion is a bike.
  • With three-year increases (2006 to 2009) of 75 percent in the number of people riding and 82 percent in bicycle miles, cycling in New York is booming like never before.

The last finding comes directly from the DOT’s screenline counts (PDF, see page 2), which are now taken on ten different days and show an average increase of 79 percent for the same three-year period. My estimates of 75 percent more NYC cyclists and 82 percent more city cycling pivot directly from that screenline increase, and will have to stand as a proxy for overall growth in cycling until the DOT broadens its bicycle counts.

I regret waiting so long to update the assumptions in my model, and I apologize for sowing confusion. But I’m heartened by the attention. We used to say, in the bygone days of bicycle advocacy, that a guy taking a bike ride could invent a cure for cancer and the “b-word” would keep the story out of the papers. Now the world is interested in bicycling quantification, and I’m delighted.

Can the model be improved? I’m sure it can. I invite you to take a close look at my algorithm, and I look forward to comments and suggestions. Of course, the best response would be for DOT to expand its counts geographically, not to mention seasonally. Part of mainstreaming cycling, as we termed it in the Bicycle Blueprint’s subtitle, is removing the guesswork from estimating it.

Until then, or until a better extrapolative model appears, I offer my estimate for 2009 of approximately 200,000 daily cyclists as rough but reasonable. And I believe the three-year increase rate of 75-80 percent is solid. The boom in NYC cycling is real.

  • Charles,
    I’ve always respected your work and your commitment to the cause, but this bad-math mess you’ve made is a doozy. The lesson learned should have nothing to do with variables or spreadsheets or models, but rigor and transparency and accuracy. You fought for years to get the b-word into the papers. Now that it’s there, I urge you to help keep its good name. When the bright lights of media attention shine on the forces of change, the highest standard of accuracy and honesty is a must. This is as true for the climate change scientists at East Anglia as it is for engineers and economists and authors in New York.
    Respectfully,
    Graham

  • Thanks so much for the refined analysis and especially the ancillary statistics, like:

    Estimated bicycle miles traveled are now (in 2009) 2.5 percent as great as motor vehicle miles traveled in the five boroughs. Based on that estimate, and averaged across the city’s streets, highways and greenways, 1 out of 41 vehicles in motion is a bike.

    As well as:

    Three-year increases (2006 to 2009) of 82 percent in bicycle miles.

    At the current rate of increase, bicycle miles could be 5% of total VMT traveled in New York City by the end of 2012.

    I know some community board members fond of dismissing cyclists as a meaningless “1% of the people” who need to be educated with numbers like these.

  • Graham — I appreciate the expressions of respect in your comment (#1). But I’m having trouble grasping the point of your urgings about transparency and honesty. As soon as I realized I had made a mistake by not updating key ratios in my old model, I set about revamping it… and then came clean in my post. Rather than meriting a lecture — even one conveyed with honey — I think my conduct has been precisely what you’re imploring me to do!

  • Larry Littlefield

    Looking to the future, the only way to measure an activity as diffuse as cycling is with a stratified household surveys. Possibily door to door, to capture the immigrant cyclists without land phone lines.

    The city mobilized volunteers to count trees and homeless people. Perhaps it and the advocacy groups can do the same to count bicycles:

    Owned.

    Used (how many times) in the past week for recreation.

    Used (how many times) in the past week to travel to work, school, shopping or other destinations.

    Used (how many times) in the past week as part of a job, for deliveries, as a bike messenger, provide a service.

  • WM Forester

    Why do you think DOT’s screenline counts, which are heavily weighted towards bike crossings of the East River Brides, which connect areas with high cycling activity, can be extrapolated to the entire city?

  • J:Lai

    First of all, Charles, thank you for your diligent efforts to quantify bicycle use. This is important work, and despite the difficulty of obtaining precise estimates, it provides useful information. I very much sympathize with the challenges of coming up with the meaningful numbers when you have scarce data.

    I think using the screenline count can provide information about the general direction and magnitude of trends in bicycling, but not much more. To really push for investment in infrastructure, or new legislation, it will be necessary to collect better data.

    When there is a lot of reliance on non-observable model parameters, even when they represent the best use of available data, it is too easy for opponents to attack the specific assumptions and dodge the underlying issues.

    Larry’s suggestion represents the other end of the spectrum, where you would get some very good data but it would be expensive to collect.

    Something in the middle might involve having people count the number of bikers passing through multiple points (major bridge entrance/exits, some points along major bike paths, etc.) this doesn’t get you the level of detail about why people are using the bicycle, but it does get you some more information about how many people are using bikes.

    I also think a “crowd sourced” dataset could provide good information if enough people participated, similar to what Larry posted last week about what he observed while riding.

  • The screenline counts are great information, but they probably miss the circumferential commuters and the delivery riders. Here on Streetsblog we go to lengths to encourage our leaders to envision the possibility that bikes could substitute for local trips (the one-mile rule, I think), the kinds of trips for which New Yorkers are using buses, subways, private autos or taxis for at present. A parent who bikes his or her kids to school a half-mile away in the neighborhood won’t get counted in the screenline, but that is exactly the rider we are trying to carve out space for.

  • @WM Forester: When the only solid datum is the screenline count, you have to extrapolate from it. What I didn’t do originally but have now done in the revamped model is to reduce, over time, the ratio by which I scale from the screenline to the whole city. Indeed, that ratio for 2009 is 10% less than for 2006. That reduction, which is a product of deeper-level assumptions in the model, is meant to reflect the trend you pointed out.

    @Jonathan: Of course the screenline misses circumferential trips, delivery trips, caregiver trips, etc. That’s why I had to develop a procedure to extrapolate beyond it.

  • When the only solid datum is the screenline count, you have to extrapolate from it.

    No, you can just say, “We don’t have enough data. Get us some more.” There’s no shame in not knowing.

  • kaja

    On my way in today on the Manhattan, a man was sitting in a chair on the Brooklyn side, with a cylindrical metal counter, which I believe is used to tick passing traffic.

    He was asleep.

  • Charles, the figures in row 29 of the “Borough” tab are, as far as I can tell, estimates by you of the number of trips that don’t end in the CBD.

    What I point out in my comment above is that one part of the Streetsblog platform is more neighborhood cycling.

    Let’s make a simple model where in year 0, there are two kinds of cyclists, commuters (who cross the screenline) and deliverymen (who do not). In year 0, those two groups are equal in number, each with 900 people, and the deliverymen make 40 trips per day, the commuters make two. A total of 37,800 trips. In year 1, after heavy advocacy efforts, a third group has started to cycle, families of commuters. Each family has two members, a parent and a child. Together, they rack up six trips a day. The parents make two round trips to school, and the kids make one trip. Let’s say that 10% of commuters’ families do this. So in year 1, to the 1000 inbound screenline trips, we add 600 neighborhood trips. The year 1 total is therefore 2000 + 600 + 40000, or 42,600. So in one year in my little model, the number of screenline trips has gone up 11%, but the number of neighborhood trips has gone up 12.8%.

    This kind of advocacy would be a success, I believe.

    Your model, however, doesn’t account for this kind of growth, as far as I can tell. So what good is your model to me as an advocate, when it doesn’t measure the kind of cycling I want to increase?

    My hypothesis is that advocacy has increased the number of neighborhood cyclists, but your model can’t be used to test that hypothesis.

  • Jonathan — Your interpretation of the ‘Boroughs’ worksheet in the bicycling spreadsheet isn’t quite right. It’s the combination of Row 29 (ratio of trips within each borough to trips entering or leaving the borough) and Row 23 (each borough’s bike mode share relative to Manhattan’s) that helps extrapolate from the screenline to the number of bike trips per borough. Row 21 plays a part, too.

    But those rows are for 1990. I changed some of those parameters — by a lot in some cases — for 2009. See Rows 42, 43 and 47.

    I regret that this crucial part of my spreadsheet is so complex. It’s far less straightforward than my BTA spreadsheet for evaluating congestion pricing. I appreciate your giving it a close look.

  • Larry Littlefield

    “On my way in today on the Manhattan, a man was sitting in a chair on the Brooklyn side, with a cylindrical metal counter, which I believe is used to tick passing traffic. He was asleep.”

    I said use volunteers to do the survey, not civil servants who can’t be fired.

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