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Behind the scenes with MBTA data.

Transportation is responsible for a significant chunk of the carbon emissions that is causing climate change. The IPCC has found that approximately one-quarter of global CO2 emissions in 2014 were from the transportation sector, and that this sector has seen faster emissions growth than any other. Public transit is one of many solutions that can help us reduce our collective transportation emissions. Trains and buses lower emissions because they can efficiently move many people at once. Additionally, the more priority (bus lanes, transit signal priority, etc.) that we are able to give buses in particular, the larger the emissions savings and the better the experience for our riders. 

The MBTA, in partnership with policy makers, municipalities, and businesses, has a very important role to play in making it easier for people to make sustainable transportation choices. One of the ways that the T works to get people out of single occupancy vehicles and onto trains and buses is through our Perq program, formerly known as the Corporate Pass Program. Through Perq, employers can offer pretax or subsidized monthly passes to their employees. Perq is a way for employers to incentivize their employees’ to replace vehicle trips with transit for their daily commute. According to AASHTO, work trips make up 19% of all person miles traveled in the U.S. However, access to a subsidized transit pass increases your likelihood of taking transit for other, non-commute trips as well. Employees and employers have pointed out that the “ease-of-use” aspect of employer-provided MBTA passes, in addition to the cost savings, also increase the likelihood that employees will use transit to commute. 

We know that mass transit has a lower carbon footprint than driving in most situations, and having convenient access to an MBTA pass can make the choice between driving and taking transit a little bit simpler. But, just how big of an effect can employer-provided transit passes actually have on emissions? To try to answer to this question, we partnered with a large education company based in Cambridge that participates in the Perq program. We looked into their employees’ transit patterns in aggregate to try to estimate just how many emissions they are potentially offsetting. 

What trips are employees making in the first place?

In order to know what emissions are being saved (or generated) through this company’s transit pass program, we first have to know which trips employees are actually making. How are they traveling? On which modes? Trip lengths and mode choices can significantly change the environmental benefits of transit.

To begin, we identified all Perq CharlieCards in use by employees in September of 2018, the last month for which we have complete, processed trip data. We ran those card numbers through our ODX model in order to identify every trip (an origin station and a destination station) that was taken that month. Our final dataset included each unique origin-destination (OD) pair, how many times that trip was taken, and whether the primary mode used was subway or bus.  

This analysis is structured to protect riders' privacy by keeping trips anonymous – our final model contains only OD pairs and modes and removes any personal CharlieCard information. 

We ran each OD pair identified by the ODX model through the Google Distance Matrix API in order to get the distance, in miles, for each trip if it was taken using transit, and then again for the driving equivalent. For example, the most common trip taken in this dataset was from Community College to Oak Grove on the Orange Line. This trip is approximately 4.6 miles on the train, but nearly double that (8.7 miles) when driving. With the ODX and API data, we are able to calculate the total number of passenger miles taken on bus, subway, and equivalent driving trips. 

The Perq program, however, does not just facilitate bus and subway trips. It also allows employers to provide their employees with Commuter Rail passes. Because the Commuter Rail does not use any automated fare collection systems, the only information that we have about how employees used Commuter Rail is how many passes were purchased for each zone. There is no way to identify particular stations, particular lines, or trip frequencies for these employees. Additionally, zone passes are not assigned specific employees in our data, so this analysis is inherently anonymous.

We do know more generally how Commuter Rail riders behave, through a variety of surveys, including a monthly panel surveys and biannual Keolis passenger surveys. While it can be difficult to relate reported behavior and actual behavior, we made assumptions about the average behavior of a Commuter Rail rider, and that employees who choose to commute on the Commuter Rail behave similarly. The vast majority of trips taken on the Commuter Rail are in the peak direction and include the terminal station of the line (either North Station or South Station). This is likely particularly true for employees given the location of their office near North Station. In addition, Commuter Rail riders that use the service for work, tend not to use transit for other trip purposes. 

In order to estimate trip distances for employees that ride Commuter Rail, we found the average transit and driving distances for all stations within a zone to their respective terminal stations. Driving mileage was calculated using the Google API, whereas we used track distances to determine the transit mileage. Take for example, the Zone 7 stations in the table below. We identified the transit and driving distances between each Zone 7 station and the terminal station on its line. From those distances, we determined the average transit and driving mileage from a Zone 7 station to downtown Boston. There were two employees who received Zone 7 Commuter Rail passes through Perq, and so we assumed that their trip lengths were equal to the zone average. 

Stop Name Terminal Station Transit Distance (Miles) Driving Distance (Miles)
Bradford North Station 32.5 37.4
Gloucester North Station 31.6 36.6
Haverhill North Station 32.9 37.1
Littleton/Rt 495 North Station 30.1 36.1
Rowley North Station 31.2 35.8
West Gloucester North Station 29.6 34.9
Attleboro South Station 31.8 39.4
Halifax South Station 28.1 35.4
South Attleboro South Station 36.8 44.5
Westborough South Station 34.0 36.0
       
  ZONE 3 AVERAGE 31.9 37.3

For these trips, we assumed that employees, on average, took the commuter rail every workday except one (in September 2018 that translates to 18 round trips). 

By the end of this process, we have calculated: 

1) total passenger miles on buses, 

2) total passenger miles on subway, 

3) total passenger miles on Commuter Rail, and 

4) total passenger miles driven for the equivalent of all transit trips. 

Total Passenger Miles, by mode  

Mode Total Estimated Passenger Miles, September 2018
Transit 30,255
Bus 3,775
Subway 16,056
Commuter Rail 10,444
Driving 41,695

How do those trips translate to emissions?

Estimating carbon emissions that result from different modes of transportation is a difficult process because there are so many confounding factors. Not only does the mode matter, but the age of a vehicle, the speed at which it is traveling, the condition of the road or track, and so much more can impact the emissions released. 

Thankfully, the Massachusetts Department of Environmental Protection has done a lot of this thinking already. MassDEP developed a carbon emissions calculator that takes into account the unique transportation landscape in Massachusetts to estimate emissions factors for each mode. The tool is slightly out of date, with some of the data being sourced from 2012. We are working to update the tool and come up with more accurate emissions factors, but as of now, the calculator is the most reliable source specific to our region. 

To get total carbon emissions, we ran the total estimated passenger miles by mode through the calculator. 

Results

After running the DEP calculator, we can compare multiple scenarios to see just how many emissions the Perq program is helping to offset. 

Spoiler Alert: Companies who encourage transit use can save A LOT of carbon emissions.  

The number we are relatively sure of is just how many passenger miles were ridden on transit, which the DEP calculator reports as having generated 9,810 pounds of CO2 in September of 2018. What we are less sure of is just how many of those trips replaced car trips. If we assume that every transit trip replaced a driving trip, we could estimate that 41,705 pounds of CO2 would have been generated by driving — this means that riders who took transit reduced their emissions by up to 76% 

Realistically, we know that not all transit trips replaced car trips. Some were likely biked, walked, already taken on transit, or not taken at all, meaning that these trips were either net neutral or actually generated emissions. Without a company specific travel survey, it is difficult to know exactly how employees behaved before the Perq pass, so we had to make some assumptions. To create a low bound estimate, we took MassDOT’s definition of a “bikeable distance,” and assumed that for every trip under six miles, the Perq program actually generated emissions. In this case, the comparison point for driving emissions is 24,977 pounds of CO2. Even in this fairly absurd low bound scenario (Was everyone who was traveling six miles or less biking or walking? Probably not.), riders who took transit still saw an overall emissions reduction of 60%. In the table below, you can see the amount of CO2 generated by driving in a variety of scenarios. 

Scenario Lbs of CO2 from driving
All trips would have happened in a car 41,705
All trips over one mile would have happened in a car 41,453
All trips over one and a half miles would have happened in a car 40,882
All trips over two miles would have happened in a car 39,970
All trips over four miles would have happened in a car 31,476
All trips over six miles would have happened in a car 24,977

What is a pound of CO2 after all? Are these savings very big? Try putting in some of these values into the EPA emissions equivalencies calculator below. Remember that these savings are for just one company in one month – the Perq program works with approximately 1,500 companies and is constantly growing. 

Conclusion

Transportation is the biggest contributor to carbon emissions in the state of Massachusetts, contributing 43% of the state’s total emissions, a share higher than the US and global averages. Replacing as many single occupancy vehicle trips by transit or active modes is one of the most effective ways we can reduce our carbon emissions. Employers have an important role to play in reducing the carbon footprint of commuters, and many who currently partner with the MBTA are thinking of innovative solutions to do just that. More information about the Perq program – one of many solutions for employers looking to lower their carbon footprint – is available here