The Boston rapid transit network has the shape of the hex symbol, #. In Downtown Boston, the two north-south legs are the Green Line on the west and the Orange Line on the east, and the two east-west legs are the Red Line on the south and the Blue Line on the north. The Orange and Green Lines meet farther north, but the Red and Blue Lines do not. The main impact of this gap in systemwide connectivity is that it’s really hard to get between areas only served by the Blue Line, i.e. East Boston, and ones only served by the Red Line, i.e. Cambridge, Dorchester, and Quincy. However, there is a second impact: people who do transfer between the Red and Blue Lines overload one central transfer point at Park Street, where the Red and Green Lines meet. This way, the weak connectivity of the Boston rapid transit network creates crowding at the center even though none of the individual lines is particularly crowded in the center. The topic of this post is then how crowding at transfer points can result from poor systemwide connectivity.
The current situation in Boston
Connecting between the Blue and Red Lines requires a three-seat ride, with a single-stop leg on either the Orange or Green Line. In practice, passengers mostly use the Green Line, because the Orange Line has longer transfer corridors.
Travel volumes between East Boston and Cambridge are small. Only 1,800 people commute from East Boston, Winthrop, and the parts of Revere near the Blue Line to Cambridge, and only 500 commute in the other direction. I don’t have data on non-work travel, but anecdotally, none of the scores of Cantabrigians I know travels to the Blue Line’s service area except the airport, and to the airport they drive or take the Silver Line, and moreover, only two people moved from Cambridge or Somerville to the area, a couple that subsequently left the region for Bellingham. Travel volumes between East Boston and the southern legs of the Red Line are barely larger: 1,200 from East Boston to Dorchester, Mattapan, and Quincy, 1,600 in the other direction, most likely not taking public transit since cars are a good option using the Big Dig.
Nonetheless, this small travel volume, together with connections between East Boston and South Station or Dorchester, is funneled through Park Street. According to the 2014 Blue Book, which relies in 2012 data, transfer volumes at Park Street are 29,000 in each direction (PDF-p. 16), ahead of the Red/Orange connection at Downtown Crossing, where 25,000 people transfer in each direction every weekday. Riders connecting between the Blue and Red Lines are a noticeable proportion of this volume – the East Boston-Cambridge connection, where I believe the transit mode share is high, is around 8% of the total, and then the East Boston-Dorchester connection would add a few more percentage points.
Why Soviet triangles exist
In a number of metro networks, especially ones built in the communist bloc, there are three lines meeting in a triangle, without a central transfer point. This is almost true of the first three subway lines in Boston, omitting the Red Line: they meet in a triangle, but the Green and Orange Lines do not cross, whereas in true Soviet triangles lines meet and cross.
The reason for this typology rather than for the less common one in which all three lines meet at one station, as in Stockholm, is that it spreads transfer loads. Stockholm’s transfer point, T-Centralen, has 184,000 daily boardings (source, PDF-p. 13), almost as many as Times Square, which is served by 14 inbound tracks to T-Centralen’s 5 and is in a city with 5.6 million weekday trips to Stockholm’s 1.1 million. Urban transit networks should avoid such situations, which lead to central crowding that is very difficult to alleviate. Adding pedestrian circulation is always possible, but is more expensive at a multilevel central station than at a simple two-line crossing.
The triangle is just a convenient way of building three lines. As the number of lines grows beyond three, more connectivity is needed. Moscow’s fourth line, Line 5, is a circle, constructed explicitly to decongest the central transfer station between the first three lines. More commonly, additional lines are radials, especially in cities with water constraints that make circles difficult, like Boston and New York; but those should meet all the older radii, ideally away from existing transfer stations in order to reduce congestion. When they miss connections, either by crossing without interchange or by not crossing at all, they instead funnel more cross-city traffic through the existing transfer points, increasing ridership without increasing the capacity required to absorb it.
The way out
The situation is usually hard to fix. It’s much harder to fix missed connections, or parallel lines that diverge in both directions, than to connect two parallel lines when one of these lines terminates in city center, which Boston’s Blue Line does. The one saving grace is that cities with many missed connections, led by New York and Tokyo, also have very expansive networks with so many transfer points that individual interchanges do not become overloaded.
In large cities that do have problems with overcrowded transfer points, including London and Paris, the solution is to keep building out the network with many connections. London tries to weaken the network by reducing transfer opportunities: thus, Crossrail has no connection to Oxford Circus, the single busiest non-mainline Underground station, in order to prevent it from becoming any more crowded, and the Battersea extension of the Northern line deliberately misses a connection to the Victoria line. Paris has a better solution – it invests in circumferential transit, in the form of Metro Line 15 ringing the city at close distance, as well as extensions to Tramway Line 3, just inside city limits.
While the solution always involves investing more in the transit network, its precise nature depends on the city’s peculiar geography. In Paris, a compact city on a narrow river, adding more circles is an option, as is adding more RER lines so that people would be able to avoid difficult Metro-to-RER transfers. In London, the population density is too low and the construction costs are too high for a greenfield circle; the existing circle, the Overground, is cobbled together from freight bypasses and is replete with missed radial Underground connections. Thus, the solution in London has to come from radials that offer alternatives to the congestion of the Victoria line.
In Boston, a much smaller city, the Red-Blue Connector is easier since the Red and Blue Lines almost touch. It only takes about 600 meters of cut-and-cover tunnel under a wide road to continue the Blue Line beyond its current terminus in Downtown Boston and meet the Red Line at Charles-MGH; to first order, it should cost not much more than $100 million. The transfer would not be easy, since the Red Line is elevated there and the Blue Line would be underground, but it would still be better than the three-seat ride involving the Green Line. A competent state government with interest in improving transportation connectivity for its residents – that is, a government that is nothing like the one Massachusetts has – would fix this problem within a few years. Boston is fortunate in not needing painful deep tunneling under a medieval city center like London or hundreds of kilometers of inner suburban tunneling like Paris – it only needs to kick out the political bums, unfortunately a much harder task.