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Behind the scenes with MBTA data.

Here at the MBTA, we keep a close eye on ridership trends and are always working on ways to better collect and visualize ridership data. After the unfortunate derailment on the Red Line in June that drastically affected service throughout the summer, we used Tableau to explore how ridership on the line had been impacted. This post will examine some of the changes in ridership we saw in the period following the derailment as normal service was restored.

 

The Data

Readers of the blog and transit data enthusiasts will remember that “ridership” does not mean any particular measure, and the ridership we report to the public is estimated based on multiple data sources and historically-based factors. To examine the effects of the derailment, we used card and ticket validations (referred to as “taps” in this post for simplicity) at the gates of stations that serve the Red Line. The data showing taps at gates should record the majority of people passing through them reliably, are comparable to other time periods going back to 2013, and are available at a very granular level. For stations where passengers can board multiple lines (in this case, Downtown Crossing and Park Street), we used the same “split” factors that we used for the dashboard and other reporting to assign a portion of their entries to the Red Line. We did not use any factor to estimate non-interaction; our assumption throughout this post is that non-interaction was roughly static throughout the time periods examined. We also did not attempt to account for passengers who board the Red Line via transfer from another line. It seems possible that fewer passengers would transfer from other lines than usual given the reduced service levels, but we had no reasonable way to measure this.

 

To explore the data, we queried our research database for all taps on all gates, grouping the data into 30-minute periods and adding attributes for the service date (measured from 3 AM until 2:59 AM the next morning), the type of service in effect for that date (Weekday, Saturday, Sunday, or Holiday), the station and other characteristics about the taps. Once this dataset was built, we loaded it into Tableau and built some views to start exploring.

 

Service was affected differently in different parts of the Red Line. The initial derailment damaged the signal bunkers housed at JFK/UMass station and forced the Red Line to operate in manual block mode. Automatic signaling was fully restored from JFK/UMass north to Alewife on July 31, but automatic signaling was not restored on the Ashmont branch until September 11, and was not completely restored on the entirety of the Red Line until September 23. We also know that many people traveling from either Red Line branch do not frequently use the Red Line north of downtown, and similarly, that many people traveling from stations north of downtown are only going as far as downtown, or even just to Kendall. To examine how these geographically-distributed service impacts have affected ridership spatially, we divided the boardings into five groups based on area of the city. These groups were:

 

 Area  Stations Included
 Cambridge / Somerville  Alewife, Davis, Porter, Harvard,       Central, Kendall / MIT
 Downtown  Charles / MGH, Park Street,       Downtown Crossing, South Station 
 South Boston  Broadway, Andrew
 Dorchester  JFK / UMass, Savin Hill, Fields       Corner, Shawmut, Ashmont
 Quincy / Braintree  North Quincy, Wollaston, Quincy       Center, Quincy Adams, Braintree

 

The Results

To get an idea of longer-term trends on the Red Line, we put together the following chart, which shows daily weekday taps on the Red Line (all stations) over the last two years, with a 20-day moving average that smooths the data to show trends. You can see that we generally have a big dip in ridership in December (Holidays like Thanksgiving and Christmas where we run reduced service are excluded, but we see lower ridership on the weekdays surrounding them). You can also see that we generally have our highest ridership from late September through October when school is in session and there are few breaks in most people’s schedules. You can also see lower ridership in March 2018, when there were a number of storms that closed schools and otherwise affected ridership. Finally, you can see the drop in ridership over this summer likely due to the impacts of the derailment. Ridership is usually low in the week around 4th of July, and towards the end of August, but a decrease can be seen this summer right around June 11 (the day of the derailment) and while July had some higher-ridership days, the overall ridership was about 5% lower than last summer. 

 

Chart of Total Taps with Moving Average

As expected, ridership was less affected in places where service was less affected. Here are some views of the above chart, filtering to just the taps at the Cambridge / Somerville and Quincy / Braintree stations as grouped above. First, the charts show the last 15 months (showing the last two summers) with a 20-day moving average, then they show taps at the stations since May 2019, with a 10-day moving average. We chose the Quincy and Cambridge stations as they had the greatest difference in service as well as the greatest difference in ridership.

 A chart of the Quincy / Braintree branch with moving average

 A zoomed-in version of the chart of the Quincy / Braintree branch with moving average

 

 A zoomed-in version of the chart of the Cambridge section with moving average

 

A zoomed-in version of the chart of the Cambridge section with moving average

You can see from the above charts that in Cambridge and Somerville, ridership returned close to its previous levels quite soon after the derailment, and with the exception of the 4th of July week, remained at this level until the last couple weeks of August. In Quincy and Braintree, however, ridership did not rebound to the same level, and this drop continued for the remainder of the summer. 

 

We took a look at the median weekday ridership compared to the previous year in each of the areas. The time periods here are divided into three: January 1 – May 31, June 1 – August 31, and September. For September 2019, the data is complete through September 27.

 

  Change in Median Weekday Ridership
 Area  January-May   June-August   September 
 Cambridge /   Somerville  2.0%  -1.3%  -1.7%
 Downtown  1.2%  -3.0%  -2.1%
 South Boston  -1.5%  -7.8%  -4.9%
 Dorchester  0.4%  -9.3%  -3.3%
 Quincy /         Braintree  -3.9%  -11.9%  -3.1%

 

In the first 5 months of 2019, ridership was generally steady or up slightly compared to the previous year. While part of this is attributed to the low ridership in March 2018 due to snow, we chose to use the median here to mitigate the effect of such days (as well as the abnormally high ridership on February 5, 2019 due to the Patriots’ championship parade). The exception to this trend was the Braintree branch, where ridership was down nearly 4%. While Wollaston station was closed in both time periods, this is likely to due to increasing construction impacts from various projects along the branch, or perhaps due to people switching to Commuter Rail in the area.

 

After the derailment, we saw more disparate impacts. The Dorchester and Braintree branches saw the biggest drop in median ridership, likely because service was affected there the most and also because those areas have in higher levels of car ownership (in the case of Quincy residents) and more alternate routes to downtown. Since Wollaston station re-opened, we might have expected a greater increase in Quincy and Braintree; however, we looked at the data and noticed that most Wollaston riders seemed to switch to North Quincy while Wollaston was closed (ridership at Wollaston, North Quincy and Quincy Center combined did not significantly change after Wollaston reopened). Ridership in Cambridge and Somerville barely dropped at all compared to the previous summer, which is likely an effect of service being better and there being fewer alternate routes: Passengers could switch to the Commuter Rail at Porter, but if they were going somewhere for which that trip was convenient, they probably were taking the Fitchburg line already. Downtown ridership was down, but that is likely largely a product of the ridership in the other areas.

 

So far in September, we have seen ridership much closer to last September than over the summer, but in most areas, we are down a few percent. Some of this is due to missing data for the last few days of September – we tend to see higher ridership at the end of September than at the beginning. To be sure, we also took a look at the median ridership through the first 13 non-holiday weekdays of each month, as well as the averages. The medians were very close to the average ridership, and through the first 13 days, the changes between the median ridership in the two months were similar, as shown above.

 

Last month, as people returned from vacations and went back to school, ridership (as measured by taps at stations) had rebounded on the Red Line compared to the summer, and overall is down 2.5% from last September. In Cambridge and Somerville, where service was least impacted by the derailment, ridership is nearly the same as last September and was only slightly down during the summer. In Braintree and Quincy, ridership is down nearly 4 percent, but there were still significant service impacts in this area into September. In South Boston and Dorchester, ridership is also down even though service is largely restored. It is possible that usual riders may have switched to another service or mode, and either may have found that this new method serves their trip better, or may not be aware that service has been restored. We will continue to watch ridership at these stations now that full service is restored and we move into our usual high ridership month of October.