The New York City Taxi & Limousine Commission publishes summary reports that include aggregate statistics about taxi, Uber, and Lyft usage. These are in addition to the trip-level data that I wrote about previously; although the summary reports contain much less detail, they’re updated more frequently, which provides a more current glimpse into the state of the cutthroat NYC taxi market.
I’ve updated the nyc-taxi-data GitHub repository with code to fetch and process the summary reports, and you can return here for updates in the future: the graphs on this page will continue to update as the TLC releases more data.
Trips Per Day in NYC: Taxi vs. Uber vs. Lyft
The summary data includes the number of trips taken by yellow taxis and for-hire vehicles:
This graph will continue to update as the TLC releases additional data, but at the time I wrote this in April 2016, the most recent data shows yellow taxis provided 60,000 fewer trips per day in January 2016 compared to one year earlier, while Uber provided 70,000 more trips per day over the same time horizon.
Although the Uber data only begins in 2015, if we zoom out to 2010, it’s even more apparent that yellow taxis are losing market share.
Lyft began reporting data in April 2015, and expanded aggressively throughout that summer, reaching a peak of 19,000 trips per day in December 2015.
Total Vehicles on the Road
The summary reports also include the total number of vehicles dispatched by each service:
Again this graph will update in the future when more data is available, but as of January 2016 there are just over 13,000 yellow taxis in New York, a number that is strictly regulated by the taxi medallion system. Uber has grown from 10,000 vehicles dispatched per week at the beginning of 2015 to over 25,000 in January 2016, while Lyft accounts for another 5,000.
However, the Uber/Lyft numbers might not be as dramatic as they seem: the TLC’s data does not indicate how many days per week Uber/Lyft vehicles work, only the total number of trips per week and the total number of vehicles that made at least one trip.
A study by Jonathan Hall and Alan Krueger reported that 42% of UberX drivers in New York work fewer than 15 hours per week, while another 35% work 16–34 hours per week. If those numbers are true, then a very rough guess might be that about half of those 25,000 vehicles make at least one pickup on any given day.
Yellow taxi utilization rates are much higher: the TLC statistics report that the average medallion is active 29 days per month, 14 hours per day (note that multiple drivers can share a medallion).
The controversial question is whether the influx of Uber, Lyft, and other for-hire vehicles has worsened congestion problems in NYC. I’ll stay out of that kerfuffle for now, but at least the popular narrative is that the city’s study did not blame Uber for increased congestion in Manhattan.
It would be interesting to look at the trip-level taxi data to see if taxi rides from point A to point B have gotten slower over the years in various parts of the city. But even if they have, it would be difficult if not impossible to blame it on for-hire vehicles—or any other single factor—using only the trip-level taxi data.
What’s Up With Lyft?
Update 4/26/16: apparently there was some kind of data reporting error between Lyft and the TLC in January 2016, which has now been corrected. When I originally wrote this section, the graph looked like this. Based on the revised data, Lyft’s trips per day in Feb 2016 slightly exceeded its previous Dec 2015 peak.
Lyft has attempted to expand its presence in New York, and it seems like they succeeded in growing a lot during the summer of 2015. But from mid-December 2015 through the end of January 2016, if the TLC’s numbers are to be believed, Lyft’s daily usage declined by 43%:
Lyft’s number of vehicles dispatched per week also declined over the same period, though by a more modest 12%. It’s possible that the TLC’s data is incorrect, incomplete, or otherwise somehow doesn’t tell the full story, but taken at face value the data does not look good for Lyft. Note that Uber trips also declined by 30% during the 2015 Christmas/New Year weeks, but Uber usage rebounded in January, whereas Lyft usage did not.
According to figures the company provided to Re/code, Lyft has seen its driver pool grow by a factor of four and has seen six times the number of active weekly riders in New York since May 2015. In February , Lyft drivers in New York performed six times the total number of rides they performed in May 2015, the company said.
These numbers are at least roughly corroborated by the TLC summary data, although public Lyft data was only available through January 2016 at the time I wrote this. But then the article continues:
[Lyft] declined to give whole figures for how many drivers and rides it has in New York.
Unless I’m misunderstanding something, the data is a matter of public record, and anyone can go download it from the TLC’s website. If the claimed six-fold increase in rides from May 2015 to February 2016 is true, that suggests February’s Lyft data will average 18,000 trips per day, erasing all of January’s drop.
I suppose we’ll find out when the new data gets posted to the TLC’s website. Again, you can check out GitHub for scripts to automate the collection and processing of the data.
Uber’s Revenue in NYC
Uber’s revenue numbers are not publicly disclosed, but we can piece together different bits of information to arrive at a very rough estimate for Uber’s New York revenue in 2015:
The TLC data reports 36.3 million Uber trips in NYC in 2015
Uber published average NYC UberX fares for 2012–2014. The average fare was $27 in September 2014, but fares have been decreasing since 2012. Let’s guess a $25 average fare for Uber’s NYC trips in 2015, including UberX and UberBlack
By comparison, the average yellow taxi fare was a bit over $14 in 2015
Uber takes a 20–25% commission, call it 22% on average
That gives us (36.3 * $25 * 0.22) = $200 million estimated revenue for Uber in NYC in 2015.
UberX’s recent NYC fare cut will probably increase demand for rides while lowering the average fare. Simultaneously Uber might charge higher commissions, and who knows how surge pricing trends might evolve. I doubt we’ll see too many public data points surrounding revenue, but maybe there will be enough to continue the “rough estimate” game.
It will be interesting to see what happens in 2016. Like many New Yorkers, I’ll be curious to see if Uber continues to gain market share, if yellow taxis do anything to stanch their wounds, and if Lyft—or any other newcomers—can muscle their way into the ranks of the major players.
The NBA’s Stats API provides data for every single shot attempted during an NBA game since 1996, including location coordinates on the court. I built a tool called BallR, using R’s Shiny framework, to explore NBA shot data at the player-level.
BallR lets you select a player and season, then creates a customizable chart that shows shot patterns across the court. Additionally, it calculates aggregate statistics like field goal percentage and points per shot attempt, and compares the selected player to league averages at different areas of the court.
Analyze your favorite players in the app below:
Run the App Locally
It’s very easy to run the app on your own computer, all you have to do is paste the following lines into an R console:
BallR lets you choose from 3 primary chart types: hexagonal, scatter, and heat map. You can toggle between them using the radio buttons in the app’s sidebar.
Hexagonal charts, popularized by Kirk Goldsberry at Grantland, group shots into hexagonal regions, then calculate aggregate statistics within each hexagon. Hexagon sizes and opacities are proportional to the number of shots taken within each hexagon, while the color scale represents a metric of your choice, which can be one of:
FG% vs. league average
Points per shot
For example, here’s Stephen Curry’s FG% relative to the league average within each region of the court during the 2015–16 season:
The chart confirms the obvious: Stephen Curry is a great shooter. His 3-point field goal percentage is more than 11 percentage points above the league average, and he also scores more efficiently than average when closer to the basket.
Compare to another all-time great, Kobe Bryant, who has been shooting poorly this season:
Kobe’s shot chart shows that he’s shooting below the league average from most areas of the court, especially 3-point range (Kobe’s 2005–06 shot chart, on the other hand, looks much nicer).
Scatter charts are the most straightforward option: they plot each shot as a single point, color-coding for whether the shot was made or missed. Here’s an example again for Stephen Curry:
Anecdotally I’ve found that heat maps often show that most shot attempts are taken in the restricted area near the basket, even for players you might think of as outside shooters. BallR lets you apply filter to focus on specific areas of the court, and it’s sometimes more interesting to filter out restricted area shots when generating heat maps. For example here’s the heat map of Stephen Curry’s shot attempts excluding shots from within the restricted area (see here for Curry’s unfiltered heat map):
The heat map shows that—at least when he’s not shooting from the restricted area—Curry attempts most of his shots from the “Above the break 3” zone, with a slight bias to right side of that area (confusingly, that’s his left, but the NBA Stats API calls it the “Right Center” of the court)
LeBron James even more heavily shoots from the restricted area, but when we filter out those shots, we see his favorite area is mid-range to his right:
I was curious if this pattern of LeBron favoring his right side has always been so pronounced, so I took all 19,000+ regular season shots he’s attempted in his career since 2003, and calculated the percentage that came from the left, right, and center of the court in each season:
It’s a bit confusing because what the NBA Stats API calls the “right” side of the court is actually the left side of the court from LeBron’s perspective, but the data shows that in 2015–16, LeBron has taken significantly fewer shots from his left compared to previous seasons. The data also confirms that LeBron’s shooting performance in 2015–16 has been below his historical average from almost every distance:
The BallR app doesn’t currently have a good way to do these historical analyses on-demand, so I had to write additional R scripts, but a potential future improvement might be to create a backend that caches the shot data and exposes additional endpoints that aggregate data across seasons, teams, or maybe even the whole league.
Limitations of Shot Charts
There’s a ton of data not captured in shot charts, and it’s easy to draw unjustified conclusions when looking only at shot attempts and results. For example, you might look at a shot chart and think, “well, points per shot is highest in the restricted area, so teams should take more shots in the restricted area.”
You might even be right, but shot charts definitely don’t prove it. Passing or dribbling the ball into the restricted area probably increases the risk of a turnover, and that risk might more than offset the increase in field goal percentage compared to a longer shot, though we don’t know that based on shot charts alone.
Shot charts also don’t tell us anything about:
Locations of the nearest defenders
Probability of an offensive rebound after a miss
Probability that the shooter will get fouled
Next-best options at the time of the shot: was another player open for a higher value shot?
Game context: a high percentage 2-point shot is useless at the buzzer if you’re down by 3
I’d imagine that NBA analysts try to quantify all of these factors and more when analyzing decision-making, and the NBA Stats API probably even provides some helpful data at various other undocumented endpoints. It could make for another area of future improvement to incorporate whatever additional data exists into the charts.
Much like with the taxi and Uber post, I’ve split the analysis into sections, covering visualization, the relationship between cyclist age, gender, and Google Maps time estimates, modeling the impact of the weather on Citi Bike ridership, and more:
I took Citi Bike trips from Wednesday, September 16, 2015, and created an animation using the Torque.js library from CartoDB, assuming that every trip followed the recommended cycling directions from Google Maps. There were a total of 51,179 trips that day, but I excluded trips that started and ended at the same station, leaving 47,969 trips in the visualization. Every blue dot on the map represents a single Citi Bike trip, and the small orange dots represent the 493 Citi Bike stations scattered throughout the city:
If you stare at the animation for a bit, you start to see some trends. My personal favorite spots to watch are the bridges that connect Brooklyn to Lower Manhattan. In the morning, beginning around 8 AM, you see a steady volume of bikes crossing from Brooklyn into Manhattan over the Brooklyn, Manhattan, and Williamsburg bridges. In the middle of the day, the bridges are generally less busy, then starting around 5:30 PM, we see the blue dots streaming from Manhattan back into Brooklyn, as riders leave their Manhattan offices to head back to their Brooklyn homes.
We can observe this phenomenon directly from the data, by looking at an hourly graph of trips that travel between Manhattan and the outer boroughs:
Sure enough, in the mornings there are more rides from Brooklyn to Manhattan than vice versa, while in the evenings there are more people riding from Manhattan to Brooklyn. For what it’s worth, most Citi Bike trips start and end in Manhattan. The overall breakdown since the program’s expansion in August 2015:
88% of trips start and end in Manhattan
8% of trips start and end in an outer borough
4% of trips travel between Manhattan and an outer borough
There are other distinct commuting patterns in the animation: the stretch of 1st Avenue heading north from 59th Street has very little Citi Bike traffic in the morning, but starting around 5 PM the volume picks up as people presumably head home from their Midtown offices to the Upper East Side.
Similarly, if we look during the morning rush at the parallel stretches of 1st and 2nd avenues stretching from the Lower East Side through Murray Hill, there’s clearly more volume heading north along 1st Avenue heading into Midtown. In the evening there’s more volume heading south along 2nd Avenue, as workers head home to the residential neighborhoods.
If we take all trips since Citi Bike’s expansion in August 2015, and again assume everyone followed Google Maps cycling directions, we can see which road segments throughout the city are most traveled by Citi Bikes. Here’s a map showing the most popular roads, where the thickness and brightness of the lines are based on the number of Citi Bikes that traveled that segment (click here to view higher resolution):
This map is reminiscent of the maps of taxi pickups and drop offs from my previous post, but they’re actually a bit different. The taxi maps were made of individual dots, where each dot was a pickup or drop off, while the Citi Bike map above counts each trip as a series of line segments, from the trip’s starting point to its finish.
The map shows a handful of primary routes for cyclists: 8th and 9th avenues heading uptown and downtown, respectively, on the west side, and 1st and 2nd avenues heading uptown and downtown, respectively, on the east side. The single road segment most trafficked by Citi Bikes lies along 8th Avenue, from W 28th Street to W 29th Street. Other main bike routes include Broadway, cutting diagonally across Midtown Manhattan, and the west side bike path along the Hudson River.
Remember that the map and animation assume people follow Google Maps cycling directions, which is definitely not always true. Google Maps seems to express strong preference for roads that have protected bike paths, which is why, for example, 8th Avenue has lots of traffic heading uptown, but 6th Avenue has very little. Both avenues head northbound, but only 8th Avenue has a protected bike path.
Unlike taxis, Citi Bikes cannot pick up and drop off at any arbitrary point in the city. Instead, riders can pick up and drop off bikes at finite number of stations across the city. Citi Bikes haven’t reached the ubiquity of taxis—in 2015 there were likely about 175 million taxi trips, 35 million Uber trips, and 10 million Citi Bike rides—but the bike share has plans to continue its expansion in the coming years.
Citi Bike makes data available for every individual trip in the system. Each trip record includes:
Station locations for where the ride started and ended
Timestamps for when the ride started and ended
Rider birth year
Whether the rider is an annual Citi Bike subscriber or a short-term customer
A unique identifier for the bike used
Here’s a graph of monthly usage since the program’s inception in June 2013:
The system gets more usage on weekdays than on weekends, and if we look at trips by hour of the day, we can see that weekday riders primarily use Citi Bikes to commute to and from work, with peak hours from 8–9 AM and 5–7 PM. Weekend riders, on the other hand, prefer a more leisurely schedule, with most weekend rides occurring in the mid afternoon hours:
Age, Gender, and the Accuracy of Google Maps Cycling Time Estimates
The age and gender demographic data can be combined with Google Maps cycling directions to address a host of interesting questions, including:
How fast do Citi Bike riders tend to travel?
How accurate are Google Maps cycling time estimates?
How do age and gender impact biking speed?
For each trip, we’ll proxy the trip’s average speed by taking the distance traveled according to Google Maps, and dividing by the amount of time the trip took. This probably understates the rider’s actual average bike speed, since the trip includes time spent unlocking the bike from the origin station, adjusting it, perhaps checking a phone for directions or dealing with other distractions, and returning the bike at the destination station.
Additionally, it assumes the rider follows Google Maps directions. If the rider actually took a longer route than the one suggested by Google, that would be more distance traveled, and we would underestimate the average trip speed. On the other hand, if the rider took a more direct route than suggested by Google, it’s possible we might overestimate the trip speed.
We have no idea about any individual rider’s intent: some riders are probably trying to get from point A to point B as quickly as safely possible, while others might want to take a scenic route which happens to start at point A and end at point B. The latter group will almost certainly not follow a direct route, and so we’ll end up calculating a very slow average speed for these trips, even if the riders were pedaling hard the entire time.
Accordingly, for an analysis of bike speed, I restricted to the following subset of trips, which I at least weakly claim is more likely to include riders who are trying to get from point A to point B quickly:
Weekdays, excluding holidays
Rush hour (7–10 AM, 5–8 PM)
Average trip speed between 4 and 35 miles per hour (to avoid faulty data)
I then bucketed into cohorts defined by age, gender, and distance traveled, and calculated average trip speeds:
The average speed across all such trips is 8.3 miles per hour, and the graph makes clear that younger riders tend to travel faster than older riders, men tend to travel faster than women, and trips covering longer distances have higher average speeds than shorter distance trips.
It’s also interesting to compare actual trip times to estimated times from Google Maps. Google Maps knows, for example, that the average speed along a wide, protected bike path will be faster than the speed along a narrow cross street that has no dedicated bike lane. I took the same cohorts and calculated the average difference between actual travel time and Google Maps estimated travel time:
If everyone took exactly the amount of time estimated by Google Maps cycling directions, we’d see a series of flat lines at 0. However, every bucket has a positive difference, meaning that actual trip times are slower than predicted by Google Maps, by an average of 92 seconds. As mentioned earlier, part of that is because Google Maps estimates don’t account for time spent transacting at Citi Bike stations, and we can’t guarantee that every rider in our dataset was even trying to get from point A to B quickly.
I ran a linear regression in R to model the difference between actual and estimated travel time as a function of gender, age, and distance traveled. The point of the regression isn’t so much to make any accurate predictions—it’d be especially bad to extrapolate the regression for longer distance trips—but more to understand the relative magnitude of each variable’s impact:
lm(formula = difference_in_seconds ~ gender + age + distance_in_miles, data = rush_hour_data)Coefficients: Estimate Std. Error t value Pr(>|t|)(Intercept)34.030.338846100.4<2e-16genderMale -87.130.192166-453.4<2e-16age 2.250.007327306.3<2e-16distance_in_miles 25.690.072994352.0<2e-16---Residual standard error:213.6 on 7226125 degrees of freedom
Multiple R-squared:0.05475, Adjusted R-squared:0.05475F-statistic:1.395e+05 on 3 and 7226125 DF, p-value:<2.2e-16
The regression’s low R^2 of 0.055 reiterates that the data has lots of variance, and for any given trip the model is unlikely to produce a particularly accurate estimate. But the model at least gives us a simple formula to make a crude estimate of how long a Citi Bike subscriber’s rush hour trip will take relative to the Google Maps estimate:
Start with 34
If male, subtract 87
Add (2.2 * age in years)
Add (25.7 * trip distance in miles)
The result is the average number of seconds between actual and Google Maps estimated trip times, with a positive number indicating a slower than estimated trip, and a negative number indicating a faster than estimated trip. Yes, it means that for every year you get older, you’re liable to be 2.2 seconds slower on your regular Citi Bike commute route!
Anonymizing Data is Hard!
In my post about taxi data, I included a section about data privacy, noting that precise pick up and drop off coordinates might reveal potentially sensitive information about where people live, work, and socialize. Citi Bike data does not have the same issues with precise coordinates, since all Citi Bike trips have to start and end at one of the 493 fixed stations.
But unlike the taxi data, Citi Bike includes demographic information about its riders, namely gender, birth year, and subscriber status. At first glance that might not seem too revealing, but it turns out that it’s enough to uniquely identify many Citi Bike trips. If you know the following information about an individual Citi Bike trip:
The rider is an annual subscriber
Their birth year
The station where they picked up a Citi Bike
The date and time they picked up the bike, rounded to the nearest hour
Then you can uniquely identify that individual trip 84% of the time! That means you can find out where and when the rider dropped off the bike, which might be sensitive information. Because men account for 77% of all subscriber trips, it’s even easier to uniquely identify rides by women: if we restrict to female riders, then 92% of trips can be uniquely identified. It’s also easier to identify riders who are significantly younger or older than average:
If instead of knowing the trip’s starting time to the nearest hour you only knew it to the nearest day, then you’d be able to identify 28% of all trips, but still 49% of trips by women.
On some level this shouldn’t be too surprising: a famous paper by Latanya Sweeney showed that 87% of the U.S. population is uniquely identified by birthdate, gender, and ZIP code. We probably have a bias toward underestimating how easy it is to identify people from what seems like limited data, and I hope that people think about that when they decide what data should be made publicly available.
Disclaimer: I know nothing about the logistics of running a bike share system. I’d imagine, though, that one of the big issues is making sure that there are bikes available at stations where people want to pick them up. If station A starts the day with lots of bikes, but people take them out to other stations and nobody returns any bikes to A, then A will run out of bikes, and that’s bad.
The bike share operator could transport additional bikes to A to meet demand, but that costs time/money, so the operator probably wants to avoid it as much as possible. The data lets us measure how often bikes “magically” transport from one station to another, even though no one took a ride. I took each bike drop off, and calculated the percentage of rides where the bike’s next trip started at a different station from where the previous trip dropped off:
From July 2013 through March 2015, around 13% of bikes were somehow transported from their drop off stations to different stations before being ridden again. Since April 2015, though, that rate has decreased to about 4%. I have no idea why: my first guess was that there were more total bikes added to the system, but the number of bikes in use did not change in March 2015. There were no stations added or removed around then either, so that seems like an unlikely explanation. Maybe the operator developed a smarter system to allocate bikes, which resulted in a lower transfer percentage?
Different neighborhoods have different transfer patterns, too. Bikes dropped off in Manhattan’s East Village have a much higher chance of being transported if they’re dropped off in the evening:
While transfers are more likely in Fort Greene, Brooklyn for bikes dropped off in the morning:
And in Midtown, Manhattan, drop offs at morning or evening rush hour are more likely to be transported:
Add it all up and I’m not exactly sure what it means, but it seems like something that could be pursued further. The Citi Bike program has plans to continue its expansion in 2016, I wonder how the new stations will impact the transport rate?
Quantifying the Impact of the Weather on Citi Bike Activity
We saw earlier that there are many more Citi Bike rides in the summer than in the winter. It’s not surprising: anyone with a modicum of common sense knows that it’s not very pleasant to bike when it’s freezing cold. Similarly, biking is probably less popular on rainy and snowy days. This got me wondering: how well is Citi Bike’s daily ridership predicted by the weather?
I downloaded daily Central Park weather data from the National Climatic Data Center and joined it to the Citi Bike data in an effort to model the relationship between Citi Bike usage and the weather. The weather data includes a few variables, most notably:
Daily max temperature
Daily snow depth
Even before I began investigating the data, I suspected that a linear regression would not be appropriate for the weather model, for two main reasons:
Our dependent variable, total number of trips per day, is by definition positive. A standard linear regression can’t be guaranteed to produce a positive number
The relationship between bike rides and the weather is probably nonlinear. For example, I’d guess the change in ridership between 40 degree and 60 degree days is probably a larger magnitude than the change in ridership between 60 degree and 80 degree days
We could use a linear model with log transformations to deal with problem 1, but even then we’d be stuck with the nonlinearity issue. Let’s confirm though that the relationship between weather and ridership is in fact nonlinear:
This graph makes it pretty clear that there’s a nonlinear relationship between rides and max daily temperature. The number of trips ramps up quickly between 30 and 60 degrees, but above 60 degrees or so there’s a much weaker relationship between ridership and temperature. Let’s look at rainy days:
And snowy days:
Rain and snow are, not surprisingly, both correlated with lower ridership. The linearity of the relationships is less clear—there are also fewer observations in the dataset compared to “normal” days—but intuitively I have to believe that there’s a diminishing marginal effect of both, i.e. the difference between no rain and 0.1 inches of rain is more significant than the difference between 0.5 and 0.6 inches.
For the nonlinear regression, we first need to specify the form of the model, which I chose to look like this:
The d variables are known values for a given date d, β variables are calibrated parameters, and the capitalized functions are intermediaries that are strictly speaking unnecessary, i.e. we could write the whole model on a single line, but I find the intermediate functions make things easier to reason about. Let’s step through the model specification, one line at a time:
dtrips is the number of Citi Bike trips on date d, the dependent variable in our model. We’re breaking trips into two components: a baseline component, which is a function of the date, and a weather component, which is a function of the weather on that date.
The Baseline(d) function uses an exponent, which guarantees that it will produce a positive output. It has 3 calibrated parameters: a constant, an adjustment for days that are non-holiday weekdays, and a fudge factor for dates in the “post-expansion era”, defined as after August 25, 2015, when Citi Bike added nearly 150 stations to the system.
The Weather(d) function uses every mortgage prepayment modeler’s favorite formula: the s-curve. I readily admit I have no “deep” reason for picking this functional form, but s-curves often behave well in nonlinear models, and the earlier temperature graph kind of looked like an s-curve might fit it well.
The input to the s-curve, WeatherFactor(d), is a linear combination of the maximum temperature, precipitation, and snow depth on date d.
Raising the daily max temperature from 40 to 60 degrees increases ridership by 12,100 trips, while raising the temperature from 60 to 80 degrees increases ridership by 7,850 trips:
1 inch of rain has the same effect as decreasing the temperature by 24 degrees
1 inch of snow on the ground has the same effect as decreasing the temperature by 1.4 degrees.
In order to assess the model’s goodness of fit, we’ll look at some more graphs, starting with a scatterplot of actual vs. predicted values. Each dot represents a single day in the dataset, where the x-axis is the actual number of trips on that day, and the y-axis is the model-predicted number of trips:
The model’s root-mean-square error is 4,138, and residuals appear to be at least roughly normally distributed. Residuals appear to exhibit some heteroscedasticity, though, as the residuals have lower variance on dates with fewer trips.
The effect of the “post-expansion” fudge factor is evident in the top-right corner of the scatterplot, where it looks like there’s an asymptote around 36,000 predicted trips for dates before August 26, 2015. Ideally we’d formulate the model to avoid using a fudge factor—maybe by modeling trips at the individual station level, then aggregating up—but we’ll conveniently gloss over that.
We can also look at the time series of actual vs. predicted, aggregating to monthly totals in order to reduce noise:
I make no claim that it’s a perfect model—it uses imperfect data, has some smelly features and omissions, and all of the usual correlation/causation caveats apply—but it seems to do at least an okay job quantifying the impact of temperature, rain, and snow on Citi Bike ridership.
As always, there are still plenty more things we could study in the dataset. Bad weather probably affects cycling speeds, so we could take that into account when measuring speeds and Google Maps time estimates.
I wonder about modeling ridership at the individual station level, especially as stations are added in the future. Adding a new station is liable to affect ridership at existing stations—and it’s not even clear whether positively or negatively. A new station might cannibalize trips from other nearby stations, which wouldn’t increase total ridership by very much. But it’s also possible that a new station could have a synergistic effect with an existing station: imagine a scenario where a neighborhood with bad subway access gets a Citi Bike station, then an existing station located near the closest subway might see a surge in usage.
There are also probably plenty of analysis that could be done comparing Citi Bike data with the taxi and Uber data: what neighborhoods have the highest and lowest ratios of Citi Bike rides compared to taxi trips? And are there any commutes where it’s faster to take a Citi Bike than a taxi during rush hour traffic? Alas, these will have to wait for another time…