data science • R • data wrangling • nextbus

How accurate is Next Bus II: wrangling data

In a the previous post we extracted Next Bus predictions from the Wmata API.
If that’s too much or uninteresting to you, start here. Our extracted data looks something like this:

time                            Minutes  VehicleID  DirectionText                  RouteID  TripID
2014-08-04 18:43:33.525000      17       6506       South to Federal Triangle      64       6464186
2014-08-04 18:43:33.525000      37       7228       South to Federal Triangle      64       6464187
2014-08-04 18:43:33.525000      56       7235       South to Federal Triangle      64       6464188
2014-08-04 18:43:33.525000      80       7231       South to Federal Triangle      64       6464189
2014-08-04 18:43:43.868000      17       6506       South to Federal Triangle      64       6464186
2014-08-04 18:43:43.868000      37       7228       South to Federal Triangle      64       6464187

Goal: determine how accurate Next Bus predictions are

I did this analysis in R, however it could be easily be done in Python or one of many languages. It could even be done in JavaScript. However, doing some data magic in R first will improve performance of the d3 visualizations. It also allows for much more rapid prototyping, exploration and analysis.

Step 1: Create a timeseries

Wmata conveniently provides with us a TripID for each unique bus trip. The only problem is they’re not actually unique. The simplest fix I found was the create a unique identifier for bus trips by concatenating TripID and VehicleID. There are certainly more complex ways that use a time dimension to determine unique trips, but this worked for me.

## reading in Nextbus data
df <- read.delim('/data/bus64_8Aug2014.txt', sep='|', header=T, stringsAsFactors=F)

## setting up dates
df$time <- as.POSIXct(df$time, origin='EST')
df$hour <- as.numeric(strftime(df$time, '%H'))

## creating unique ID for bus trips
df$TripID <- as.character(df$TripID)
df$ID <- paste(df$TripID, df$VehicleID, sep="_")
df <- df[order(df$ID, df$time),]

Step 2: Flag arrivals and departures

Next Bus doesn’t actually tell us when a bus arrives. We need to determine this from the time series we collect. This is one of the reasons I collect predictions from the API every 10 seconds. There is probably a more computationally efficient method for doing this using vectorized functions, but this works fine.

## marking arrival and departure dates for Trips
depart <- by(df$time, df$ID, min)
arrive <- by(df$time, df$ID, max)
df$departure <- 0; df$arrival <- 0; df$est <- 0
for(i in 1:nrow(depart)-1) {
  tname <- names(depart)[i]
  df$departure[df$ID==tname & df$time==depart[i]] <- 1
  df$arrival[df$ID==tname & df$time==arrive[i]] <- 1
}

This gives us something like this:

time                 Minutes  VehicleID  DirectionText              RouteID  TripID   hour  ID            departure  arrival
2014-08-12 12:35:36  1        6470       South to Federal Triangle  64       6464164  12    6464164_6470  0          0
2014-08-12 12:35:46  1        6470       South to Federal Triangle  64       6464164  12    6464164_6470  0          0
2014-08-12 12:35:57  0        6470       South to Federal Triangle  64       6464164  12    6464164_6470  0          0
2014-08-12 12:36:38  0        6470       South to Federal Triangle  64       6464164  12    6464164_6470  0          0
2014-08-12 12:36:48  0        6470       South to Federal Triangle  64       6464164  12    6464164_6470  0          0
2014-08-12 12:36:58  0        6470       South to Federal Triangle  64       6464164  12    6464164_6470  0          1
2014-08-07 10:56:06  98       6482       South to Federal Triangle  64       6464164  10    6464164_6482  1          0
2014-08-07 10:56:16  98       6482       South to Federal Triangle  64       6464164  10    6464164_6482  0          0
2014-08-07 10:56:26  98       6482       South to Federal Triangle  64       6464164  10    6464164_6482  0          0

Assumption 1: When Next Bus says a bus is arriving (Minutes==0) multiple times, I take the latest prediction as the actual arrival time. In other words, I take the last prediction where Minutes==0 before the bus disappears off your Next Bus app as the arrival time.

Step 2: Filter

Assumption 2: Remove bus trips that never arrive. There are some ghost buses out there. It’s impossible to determine the error in a prediction if you don’t know the outcome (true arrival time), so these trips are removed.

# removing buses that never arrive 
TripsThatArrive <- df$ID[df$arrival==1 & df$Minutes==0]
df <- df[df$ID %in% TripsThatArrive,]

Step 3: Calculate error in Next Bus predictions

Now that we know the arrival times for each bus trip (df$arrival), the prediction in minutes until arrival (df$Minutes) and the time the prediction was made (df$time), we can figure calculate the prediction error (df$err) and actual time until arrival (df$est).

# finding actual prediction-to-arrival times
df$est <- 0
for(i in 1:nrow(depart)) {
  tname <- names(depart)[i]
  df$est[df$ID==tname] <- (df$time[df$ID==tname & df$arrival==1] - df$time[df$ID==tname])/60
}

# finding prediction errors
df$err <- df$est - df$Minutes

Step 4: Write out data to csv

Easy enough. This is the data that will feed the d3 visualizations built in the next post.

write.table(df, file='/Users/ajb/Documents/github/nextbus/data/cleanTrips.csv', row.names=F, sep=',')