Optimizing Green Line T Stops
The Green Line seems to stop too frequently. Presuming the best location for a stop is where one doesn't already exist and people would most use one, we think analyzing the popularity of stops against the distance to alternatives can help decide which existing stops are most valuable and which can be omitted. To make such a determination, we plan to use a variant of the k-means clustering algorithm that also takes into account a given weight for each point. The weight, in this case, will be a function of the stop's popularity and the proximity to the nearest alternative.
By using k-means and setting k to be x fewer than the number of existing stops, we can determine which x stops are best to remove from existing routes: the k stops nearest the cluster means (and only one stop per mean) would be the ones to persist.
To do this, we need to know the walking distances to nearest T stop alternatives, as well as the popularity of each stop.
As a metric to (hopefully) help establish our utility measurement as a desirable and positive one, we also calculate the correlation of that utility score against the availability of handicap access at stops.
These metrics provide a strong starting point to determining which Green Line T stops are most valuable to the commuting community.
The Data Sets Involved
1. GPS locations of T stops
This dataset uses information provided by the MBTA, which gives the exact GPS locations of each T-stop on the Green Line. This is used to help us create the derived datasets.
Dataset built from a comma-separated value file published by the MBTA. This is hosted at http://cs-people.bu.edu/sajarvis/datamech/mbta_gtfs/stops.txt
The location of the original zip file containing the data is included as a provenance entity.
2. Line associations of T stops
This dataset includes the stop-id, stop name, line, next inbound stop, and next outbound stop. We use this to get a sense of how each branch is ordered, and again to create the derived datasets. This information was provided by the MBTA.
Dataset is handcrafted based on stop IDs used by the MBTA and a map of T lines. The handcrafted data is hosted at http://cs-people.bu.edu/sajarvis/datamech/green_line_branch_info.json
The map used to handcraft this data set exists as a provenance entity, with a source of where we found it online.
3. Popularity of each Green Line stop
This dataset provides to average boarding population per day at each stop along the green line. We will assume that the number of boardings is proportional to the number of passengers that disembark. This information was provided by the MBTA, using the most recent boarding counts (2013).
Dataset handcrafted from a PDF published by the MBTA and the stop IDs included in the GPS database. The dataset is hosted at http://cs-people.bu.edu/sajarvis/datamech/green_line_boarding.json
The sources used to handcraft this JSON are included as provenance entities, with a source of where we found them online.
4. (derived) Walking distances to other stops on a branch
This derived dataset uses Google API to get the walking distances to the next nearest stop. This looks at the walking distances from the passengers current T-stop position to all other T-stop positions within that branch.
The provenance data does not list each URL queried, since they change for every combination of source and destination and are dependent on the combined data being used. We instead insert placeholders for the coordinates, e.g. "<source_lat>" for the source latitude.
NOTE: This will take a long time to generate because Google's API throttles the number of requests we can make (roughly 45 minutes running time currently). If we get throttled, we'll exit with an error message and non-zero code (1).
5. (derived) Time to nearest neighbor stop on same branch
This derived dataset holds information regarding the nearest neighbor stop on the same branch, including the time it takes to get there from the previous stop. If the two stops are really close together, then that means that the current stop is less important and could potentially be removed.
6. (derived) Utility measurement based on passenger saved time
In this derived dataset, we create a measurement called "people-seconds" to gauge the utility of each stop. This is a score that uses both popularity and walking time to the next nearest stop, and provides a weight for each stop. A low score is good; this means that the stop is valued and saves the greatest amount of time for the collective commuting group. This score will let us perform a weighted k-means to find the optimal k stops.
7. (derived) Normalized utility measurement
The people second measure is not directly usable as a weight to k-means, this translates it into a measure directly usable by the k-means optimization.
8. (derived) Weighted k-means optimization for each branch of Green Line
Stores the coordinates of the optimal stops, as computed by k-means, for varying values of k on each branch of the Green Line.
9. (derived) Correlation and p-value of utility ratings vs. handicap access of stops
Correlation coefficient and p-value of the correlation between our utility score (ppl seconds) and the availability of wheelchair access at each stop.
NOTE: Requires the scipy library for Python3.
The visualizations are viewable in a web browser. These visualizations need access to the database from the browser, which can't be directly obtained. So we use a small Flask application to act as a middle-man to facilitate this access. This requires flask.
1 - With the mongo daemon running, start the web app for the database API (
pip3 install flask if not already available).
cd vis/; python3 restful.py
2 - Point your favorite browser to 127.0.0.1:5000/stops.
2 - Then check out 127.0.0.1:5000/utility.
3 - Finally, 127.0.0.1:5000/meta displays some meta information about the database. It is not related directly to our measurements, but examines all collections in the database and evaluates their physical size on disk, entry count, and relates their provenance label. Relies on some database string parsing, may not be correct in all situations.