How much has COVID cost the NYC Subway system in "lost fares"?

Libraries

### Data Manipulation Packages
library(timetk)
library(tidyverse)
library(scales)
library(lubridate)

# Modeling Ecosystem
library(modeltime) 
library(tidymodels) 
library(treesnip) 

### Model Packages
library(catboost)
library(prophet)

Data

For this project, I’ll be using the MTA’s weekly fare data which contains the number swipes for each fare type, for each station. I’d previously scraped data from this website in a prior blog post so I won’t go through the methodology again.

Since for this project I don’t need station level or fare type granularity, I’m going to aggregate the data set to the date level.

dt <- readRDS('data/mta_data.RDS') %>% 
  group_by(week_start) %>% 
  summarize(fares = sum(fares))

Methodology

The dataset contains the weekly number of subway swipes from May 2010 through June 2022. To determine the number of “lost fares”, I’m going to build a forecast of the number of swipes from 2020 onwards and use the residuals between the forecast and the actual data to determine the number of “lost swipes”. Since I don’t reasonably expect a model to accurately predict 2020 onwards but I want to ensure I will have a reasonable model, I will train the model on data from 2010 through 2018 and then validate based on the 2019 data which should be similar to 2018.

Based on the validation set I will choose the best model and then using that model I will forecast 2020, 2021, and 2022.

Ultimately this test plan looks as follows:

dt %>% 
  mutate(lbl = case_when(
    week_start < ymd(20190101) ~ "a) Train",
    year(week_start) == 2019 ~ 'b) Validate',
    year(week_start) >= 2020 ~ 'c) Test'
  ), 
  total_fares = fares) %>% 
  ggplot(aes(x = week_start)) + 
  geom_line(data = dt, aes(y = fares), color = 'grey60') + 
  geom_line(aes(y = fares, color = lbl)) + 
  labs(title = 'Testing Plan for Forecasting',
       x = "Date", y = "# of Metrocard Swipes",
       color = "") + 
  scale_y_continuous(labels = comma) + 
  facet_wrap(~lbl, nrow = 3) + 
  cowplot::theme_cowplot()

In order to split the data, I’m going to first chop off the 2020+ data into a test dataframe:

test <- dt %>% filter(year(week_start) >= 2020)

And then use timetk’s time_series_split to create the sets that will be used for model development and validation:

splits <- time_series_split(
  dt %>% filter(year(week_start) < 2020) %>% arrange(week_start),
  assess = 52, cumulative = T)

The assess options tells the function to split the last 52 weeks of data into the validation set and the cumulative option tells the function to use all the other data in the training set.

The training data runs from 2010-05-29 to 2018-12-29 and the validation data runs from 2019-01-05 to 2019-12-28.

Modeling

The modeling process will use the recipe / workflow process that is used in the tidymodels ecosystem. However, add-on packages like modeltime and treesnip will allow for extensions to time series and other ML algorithms. For a more detailed look at Tidymodels check out my post on icing the kicker.

rec <- recipe(fares ~ ., data = training(splits)) %>%
  update_role(week_start, new_role = 'id') %>% 
  step_timeseries_signature(week_start) %>% 
  step_rm(matches("(.iso$)|(am.pm$)|(.xts$)|(hour)|(minute)|(second)|(wday)")) %>% 
  step_dummy(all_nominal(), one_hot = TRUE)

catboost_wf <- workflow() %>% 
  add_model(
    boost_tree(mode = 'regression') %>% 
      set_engine('catboost')
  ) %>% 
  add_recipe(rec) %>% 
  fit(training(splits))

xgboost_wf <- workflow() %>% 
  add_model(
    boost_tree(mode = 'regression') %>% 
      set_engine('xgboost')
  ) %>% 
  add_recipe(rec) %>% 
  fit(training(splits))

arima_boosted_wf <- workflow() %>% 
  add_model(
    arima_boost() %>%
      set_engine(engine = "auto_arima_xgboost")
  ) %>%
  add_recipe(rec %>% update_role(week_start, new_role = "predictor")) %>%
  fit(training(splits))


ets_wf <- workflow() %>% 
  add_model(
    exp_smoothing() %>%
      set_engine(engine = "ets")
  ) %>%
  add_recipe(rec %>% update_role(week_start, new_role = "predictor")) %>%
  fit(training(splits))

prophet_wf <- workflow() %>%
  add_model(
    prophet_reg(seasonality_yearly = TRUE) %>% 
      set_engine(engine = 'prophet')
  ) %>%
  add_recipe(rec %>% update_role(week_start, new_role = "predictor")) %>%
  fit(training(splits))

prophet_boost_wf <- workflow() %>%
  add_model(
    prophet_boost(seasonality_yearly = TRUE) %>%
      set_engine('prophet_xgboost')
  ) %>% 
  add_recipe(rec %>% update_role(week_start, new_role = "predictor")) %>%
  fit(training(splits))

calibration_table <- modeltime_table(
  catboost_wf,
  xgboost_wf,
  arima_boosted_wf,
  ets_wf,
  prophet_wf,
  prophet_boost_wf
) %>% 
  modeltime_calibrate(testing(splits))

calibration_table %>%
  modeltime_accuracy() %>%
  arrange(rmse) %>% 
  select(.model_desc, where(is.double)) %>%
  mutate(across(where(is.double), 
                ~if_else(.x < 10, round(.x, 2), round(.x, 0)))) %>%
  kable()

calibration_table %>% 
    select(.model_desc, .calibration_data) %>% 
    unnest(cols = c(.calibration_data)) %>% 
    filter(year(week_start)==2019, .model_desc != 'ACTUAL') %>% 
    ggplot(aes(x = week_start)) + 
      geom_line(aes(y = .actual), color = 'black', lty = 2) + 
      geom_line(aes(y = .prediction, color = .model_desc), lwd = 1.2) + 
      facet_wrap(~.model_desc, ncol = 2) + 
      scale_color_discrete(guide = "none") +
      scale_y_continuous(label = comma) + 
      labs(title = "Comparing Models to Test Set of 2009", 
           subtitle = "Dashed Line is Actuals",
           y = "# of Fares",
           x = "Date") + 
      theme_bw() + 
      theme(
        axis.text.x = element_text(angle = 60, hjust = .5, vjust = .5)
      )

Forecasting the COVID Time Period

Now that I’ve identified the Prophet w/ XGBoosted errors model as the best model, its time to retrain it one final time on both the training and validation data before using it to forecast the COVID time period. The refiting on all data is handled by modeltime_refit().

refit_tbl <- calibration_table %>% 
    filter(.model_desc =='PROPHET W/ XGBOOST ERRORS' ) %>%
    modeltime_refit(data = bind_rows(training(splits), testing(splits)))

Finally, the forecasting onto the test set is handled by modeltime_forecast(). The test data and actuals are passed into the function so that the actuals and forecast can be directly compared.

final_fcst <- refit_tbl %>% 
  modeltime_forecast(
    new_data = test,
    actual_data = dt,
    keep_data = TRUE
  )

The forecast vs. the actuals can be visualized with plot_modeltime_forecast():

final_fcst %>% 
  plot_modeltime_forecast(.conf_interval_show = T, .interactive = F) + 
  scale_y_continuous(labels = comma)

Calculating the “Lost Fare” Amount

Now with forecast computed I can determine the number of lost fares by comparing the forecast number of fares to the actual number of fares. Then to convert that to an amount of money, I’m using a simplistic assumption that each fare would have cost about 2 dollars. This is a heuristic since there are many different kinds of fares in the NYC Subway system which have different costs. A full-fare cost $2.75, a monthly unlimited card costs $127, for Seniors and other reduced fare populations the cost is half-price as $1.35.

loss_amt <- final_fcst %>% 
  filter(.model_desc == 'PROPHET W/ XGBOOST ERRORS',
         .index >= min(test$week_start)) %>% 
  mutate(diff = fares-.value,
         diff_lo = fares - .conf_lo,
         diff_hi = fares - .conf_hi,
         fare = diff * 2.00,
         fare_lo = diff_lo * 2.00,
         fare_high = diff_hi* 2.00) %>% 
  arrange(.index) %>%
  mutate(fares_lost = cumsum(fare),
         fares_lost_lo = cumsum(fare_lo),
         fares_lost_high = cumsum(fare_high)) 

Using the confidence intervals of the predictions I can form a range of how much in “lost fares” the MTA suffered since 2020.

Ultimately, this analysis shows that the MTA has likely lost $5B in lost fares since 2020, but it would be as low as $4.4B or as high as $5.7B.

The cumulative loss can be visualized as follows:

loss_amt %>% 
  filter(.index >= ymd(20200101)) %>%
  ggplot(aes(x = .index, y = fares_lost*-1)) + 
    geom_line() + 
    geom_ribbon(aes(ymin = fares_lost_lo*-1, ymax = fares_lost_high*-1), alpha = .3,
                fill = 'darkgreen') + 
    scale_y_continuous(labels = dollar, breaks = seq(0, 6e9, 1e9), expand = c(0 ,0)) + 
    labs(title = "Cumulative Amount of Subway Fares Lost Since 2020",
         x = "Date", y = "$ Lost", caption = "$ Lost = Projected Swipes Lost * $2.00") + 
    cowplot::theme_cowplot() + 
    theme(
      plot.title.position = 'plot',
      panel.grid.major.y = element_line(color = 'grey45')
    )

Concluding Thoughts

While things are starting to return to more “normalcy” on the NYC subway its still far from what is was in the pre-COVID times. Based on this forecasting exercise, its estimated that the MTA has already lost around $5B in “lost fares” and that number is continuing to grow. Because while things are recovering, there’s still a long way to go.