Chicago Service Cut Maps: Method
When we noticed that Chicago's CTA was planning service cuts on February 7, 2010, we realized that our maps could be very useful in comparing service before and after the cuts were implemented. We set out to make maps showing the changes in transit time into the Loop in the morning, and out of the Loop late at night. (The cuts are heaviest late at night, and lightest in the morning rush.)
Since along with the cuts will come other changes to certain transit times — the Red Line, for example, runs every 15 minutes both before and after the cuts, but is offset by 3 minutes — it is not enough to simply grab data for one fixed time. Instead, for each point in the city (on a grid of points about two blocks apart) we randomly selected six times between 7:30 and 9:30 AM. Then, using published route times, we found the start time necessary to arrive at the loop by the six times generated, on weekdays a week apart, straddling 2/7. We took the difference between the travel time before and after the cuts, for each arrival time.
This gave us six time differences for each point in our grid. For the neighborhood map, we took the mean of all the difference numbers across the neighborhood, and mapped these values. For the point map, we took a weighted average of the six values for each point and the values for its neighbors.
For the 12:30 to 2:30 AM timeframe, we used the exact same methodology, except that we measured trips outward from the Loop instead of inward.
Note: as some of you may have noticed, our maps have undergone a few last minute changes over the past few days. We've been fact-checking and refining constantly since we put the maps up, and have found several problems not in our own work, but in the publicly available trip planner data that we used to make the maps. The difficulty in getting the data to tell the right story, and do so accurately, has surprised us a bit, but we hope that the maps we have produced still shed some light on how the cuts affect different parts of Chicago. Thanks a bunch for taking a look, and hope you enjoyed our maps.