Measuring Boston's Green Line

I moved from LA to Boston about a year ago.  Since I hate driving, I was looking forward to moving somewhere with a more effective (read: existing) public transportation system.  At least, I was looking forward to it until I realized my apartment was situated on one of the green lines of the T (I recommend the 4th and 9th reviews especially).

For those of you unfamiliar with Boston’s public transit system, known as “the T”, it consists of the red, blue, and orange lines, all of which are reasonably timely subways, and then the green line.  The green line has four different branches, most of which operate at street level and, for some reason, lack any sort of tracking or timing system.

One day at work, I mentioned my disappointment with the green line to my labmates which led to a heated debate about which of the four green lines was the worst.  I was convinced the B green line was slower than the C line while my friend, Aishu, adamantly believed that the C line was slower.  One of the senior postdocs, who had been living in Boston longer than both of us, explained that the quality of the green line branches decreased alphabetically.  The B branch was the best, followed by the C, then the D, and, finally, the E.   

In spite of his expertise, I was unconvinced that his rankings were true. In an attempt to settle the matter of the T’s green line ratings and do a bit of research into the quality of Boston’s urban planning, I got Aishu to help me with an experiment.  For about six months we recorded the amount of time we spent waiting for various outbound green line trains which is arguably as valid a use of time as playing mobile phone games while waiting for the subway.

Whenever we walked into a station that served at least the B, C, and D lines, we noted the time when we got to the platform and the times at which the trains arrived until the train we were waiting for came into the station.  We also noted the crowdedness of the train car we were in.  I should point out that because the E line shares fewer stops with the B, C, and D lines than they do with each other, we do not have much data on the E line.  Additionally, most of our data comes from weekdays between 6:00 and 8:00 pm.  It is also important to note that we did not measure the average waiting time for inbound trains or how quickly the trains traveled to their destination.  Still, we ended up with enough information to test the Massachusetts Bay Transit Authority’s claims about the average wait time for green line trains.

The first thing we looked at after our recording our six months’ worth of data was the frequency of the trains.  How many trains would you have to watch go by before you finally got the one you wanted?   While not necessarily as informative as the actual waiting time, this factor still has some role in determining how irritating each green line is.  As you wait on the platform, your hopes rise every time you hear the piercing shriek of a green line train coming into the station just for it to be the wrong branch.  Repeat this experience of having your hopes dashed several times in a row, even if interval between incorrect trains is just one minute, and it’s easy to see why train frequency matters.

Our data is summarized in the figure below: 

Of course there are fewer instances of trains being the 5^th or 6^th train to go by since we stopped recording once we got on the train we were waiting for.  I was also usually waiting for the B line and recorded data more often so there are more instances of B line trains while Aishu was usually waiting for the C line.  Excluding these results, we also find that when an outbound D line train arrives in a shared station it is usually the first train to come by.  Of the 35 recorded D trains, 42% were noted to be the first train of a sequence.  This is compared with 28% of B line trains being the first to arrive, and 30% of C line trains.  In the majority of outbound trips, it seems you will only have to wait for one train to go by before the one you want arrives.

But knowing how many trains you’ll have to count before you can get on one doesn’t tell you much about how long you’ll actually have to wait.  To determine that, we present the following data:

Note: The 12 and 14 minute values are excluded due to formatting reasons and because no trains were recorded at those times.

86% of outbound B trains arrive in five minutes or fewer, as do 86% of outbound D trains; however, only 59% of outbound C trains will require you to wait fewer than five minutes.  This is true despite the fact that the single most common waiting time for a C train is 0 minutes.  1 in 4 times when you step into a station, a C train will be waiting there for you, but if you miss it you might have to wait for a while.  The longest wait time recorded was for a C train at 16 minutes, but on average you’re only going to have to wait for about five minutes (see below).

The average wait times for the other trains are shown in the following figure:

We have averages of slightly over three minutes for the B line and slightly under three minutes for the D line.  The E line appears to be on par with the C line, at around 4.5 minutes, but keep in mind we only recorded seven points of data for E line trains so the accuracy of that value is not without error.

According to the MBTA, B line commuters can expect to wait 10 minutes for a train in the evening (6:30 pm to 8:00 pm), C line commuters – seven minutes, D line commuters – 10 minutes, and E line commuters – 10 minutes.  While the average waiting times for all the lines are below the MBTA estimates, remember that the MBTA is not distinguishing between outbound and inbound trains and may list longer than expected times to be safe.  Nevertheless, it is somewhat reassuring to know that when waiting for an outbound train, you will probably be on your way in under five minutes.  Still I’m sure Bostonians would appreciate it if real-time updates were made available.

So you’ve waited your average 3.8 minutes, caught your train, and are on your way home.  How much space will you get to yourself on the ever luxurious green line?  To take into account the amount of discomfort one might have to endure on their commute, Aishu and I came up with a crowdedness scale.  It ranges from 1 – “spacious”, meaning “Wow, I practically have this train to myself!  Actually that’s kind of weird…” to 7 – “too full to get on” which I expect I don’t need to define.  In between we have 2 – “not full and not empty” indicating that there are a few people sitting, 3 – “full/normal” meaning the seats are all full and there are some people standing, but there is enough space to move around, 4 – “crowded”, a level at which you might have to stand closer to other passengers than desired, but you can still breathe, 5 – “very crowded” meaning “I’m starting to get ever so slightly uncomfortable.  I really hope nobody tries to get on at the next stop”, and 6 – “stupidly crowded” at which point you find yourself thinking “How did more people fit onto this train?  If I don’t get out of here soon I’m going to start biting everybody who’s touching me!”

As we primarily took the B and C trains, we only have crowdedness data for those two lines.  The results are as follows:

The average crowdedness of both lines were around 3 – “full/normal” although the C line tended to be slightly less crowded than the B line.  The only recorded instances of a train being too full to get on to (level 7) were with B line trains.  Never fear though, you are likely to find space on your train on either line.

Overall, it looks as if reality is actually better than what the MBTA predicts, at least for outbound trains.  On average, you will wait for less time than the MBTA website claims and you’ll have space in your train car, if not a seat.  Counter to our postdoc’s beliefs, quality of the green line branches doesn’t decrease with alphabetical order, as shown by the D line’s shortest average waiting time.  It is important to note, though, that without data on how much time you will actually spend on the train, it is still difficult to conclude whether the green line is worth using.  Considering the average speed (4.1 miles in 24:49?) of a green line train and how small Boston is, might I recommend a bicycle instead?