Two years ago, I wrote a post criticizing subway lines that mix radial and circumferential elements. These lines, for examples Shanghai Metro Lines 3 and 6 and New York’s G train before 2001, contain long radial segments, going from an outlying neighborhood toward city center, but then switch to circumferential mode, avoiding city center and instead serving secondary nodes. Such lines do not get high ridership, because they fail at either radial or circumferential transit. Recently, I was challenged in comments about my support for a mixed line that goes in the other direction: circumferential on the outside, radial on the inside. I would like to talk more about such lines.
Consider the following diagram of a subway system:
The city is shown in light gray, with its center in dark gray. There are five subway lines: the red and blue lines are straightforward radials, the green line is a straightforward circumferential, the yellow line mixes radial and circumferential as criticized in my previous post, the pink line mixes radial and circumferential in the other manner, which I will describe in this post.
The reason the yellow line is going to underperform in this system is that it fails as a radial: it does not go to city center, which has the largest concentration of destinations for transit users. People who have equal access to the red and yellow lines, north and south of city center, are much likelier to choose the red line, which takes them where they want to go. The green line fails as a radial too, but has the positive features of a circumferential: it only serves relatively nearby neighborhoods, which are likely to be denser and produce more riders per unit length; it connects to every line in the system; it allows people to connect between two radial lines without going through the congested city center; it has no dominant direction at the peak, so trains are unlikely to be full in the peak direction and empty in the reverse-peak direction. The yellow line has none of these features, unless one wants to connect between the western legs of the blue and pink lines.
The pink line still works as a radial. Its northeastern leg is a straightforward radial, but even its southwestern leg works as a radial for people who live west of the yellow line and wish to commute to city center. In this way, it is not truly a mixture of radial and circumferential elements the way the yellow line is, but is simply a radial with a circumferential element tacked on at the end.
Whether the pink line’s circumferential tail works must be evaluated against two alternatives: build nothing, and build a radial leg. This is because in an incrementally-built transit system, the radial parts of the line are typically built first, and the circumferential tail is tacked on as a later extension. In the no-build case, the pink line’s southwestern leg would simply be shorter than the other radial legs in this system. In the radial case, the pink line’s southwestern leg would look symmetric with the northeastern leg. This depends on the following factors:
- The strength of the radial alternative. If the radial alternative is strong, then this discourages building the circumferential extension, and vice versa. The radial alternative can be weak in several ways: the southwestern quadrant of the city depicted above may be already replete with radial transit and not need more; the population density in the neighborhoods that would be served by the radial option may be low; and the city’s layout may not be the above-depicted perfect circle, so that there is nowhere for the line to turn except sideways.
- The strength of the corridor that would be served by the circumferential leg. The leg can never be a complete circle, so it must be evaluated as a rapid transit line on an individual street or corridor. This far out of city center, transit demand on each route is unlikely to be high, but there may well be exceptions, for example if there is a linear secondary CBD. For example, while Seoul Metro Line 2 is fully circumferential, one of its segments follows a Tehran Avenue, a major street in Gangnam with high transit demand, which would justify a subway even if it weren’t part of a large circle.
- The strength of the circumferential transit demand from the end of the potential circumferential extension to the radial segment. In the depicted city, there may be strong demand for east-west transit south of the CBD, and the circumferential pink line is then better at serving it than connecting between the red and yellow lines via the blue line.
The original impetus for this post, as noted at the beginning, is a comment challenging me for my support of an extension of Second Avenue Subway Phase 2, going under 125th Street from the planned terminus at Lexington Avenue to Broadway, with stations at the intersection with each preexisting subway line. I contend that in this case, all three factors above point to a very strong circumferential extension. In order:
- The radial alternative is to extend Second Avenue Subway to the north, to the Bronx, presumably under Third Avenue, but according to some railfans also under University Avenue. This is problematic, for three reasons. First, the Bronx already has many north-south lines feeding into Manhattan trunk lines, with mediocre ridership. The Manhattan trunk lines are overloaded, but mostly with traffic coming from the Upper East and West Sides, Harlem, and Washington Heights. Second, Third Avenue is close to the Harlem Line, which could be used for local transit if fares and schedules are integrated with the subways and buses. And third, the plan for Second Avenue Subway is for the line to turn west at 125th toward Lexington, since 125th and Second is not as compelling a destination, and this makes it easier to extend the line to the west than to the north.
- 125th Street is a very busy street, and acts as the main street of Harlem. Transit demand is high: four bus routes use the street, with a total of 32,630 boardings per weekday on 125th Street, exclusive of other segments of those routes. This count misses people who board elsewhere and get off on 125th, but conversely assigns people who board on 125th and get off elsewhere to this street and not the other segment. But with this caveat in mind, this points to about 11,000 weekday riders per route-km, ahead of New York’s busiest bus per unit length (the M86, with about 7,000), and not far behind the subway average (15,000). This is despite the fact that, in my experience going between Columbia and the Metro-North station at Park Avenue, those buses are not faster than walking.
- East-west transit in Uptown Manhattan consists of Pokey-winning crosstown buses; the 125th Street buses are as slow on 125th. An underrated feature of Second Avenue Subway Phase 1 is that it will soon enable a two-seat subway ride from the Upper East Side to the Upper West Side, West Harlem, and Washington Heights. However, this option will require connecting at Times Square, and is useful mainly for people in the southern areas of the Upper East Side connecting to the 1/2/3 rather than to the A/B/C/D. A two-seat ride based on going up Second Avenue to 125th Street and thence connecting to the 2/3, A/B/C/D, or 1 would enable more connections, many without any backtracking. This could have a potential cascading effect on all Uptown east-west buses, and not just those using 125th Street.
Of course, a Second Avenue Subway extension on 125th Street cannot be exactly like the pink line in the diagram above, because a key feature of it is that the circumferential part is not in fact near the outer end of the city. It’s barely 5 km north of the northern edge of Midtown, not even halfway from Midtown to the northern ends of most preexisting north-south subway lines. This is how it can have such high residential and commercial density and strong transit demand. Much farther north, Fordham Road is a very strong bus corridor, with about 4,500 weekday riders per route-km on the Bx12, but this is at much higher speed than in Manhattan, about 13 km/h rather than 5 km/h. An extension of the A east toward the Bronx under Fordham would underperform, because Fordham just doesn’t have that much demand; but 125th does.
The result of this discrepancy is that in a small city, one whose subway system is only about as large as in the diagram, it’s unlikely that such circumferential extensions would work. A radial line built all the way out is going to have as its terminus either a relatively low-density area or an anchor point, such as a commercial center or big housing project, neither of which lends itself to a strong continuous circumferential corridor. A radial line built part of the way to the edge of the city could potentially find a Tehran Avenue or a 125th Street, but if the system is small, with many key outlying neighborhoods still unserved, then it is usually best to keep extending lines outward.
The factors that conspire to make a 125th Street subway extension work are in place precisely because New York already has a large, mature subway network, in which Second Avenue Subway is a relief line. Certainly the projected demand on Second Avenue is very high, but the East Side is already served by a north-south subway 500-600 meters to the west of this line; it’s being built because this subway is overcrowded, not because the East Side has no access. This means that there’s more leeway with choosing what to do with the line once it reaches Harlem – after all, the Bronx subways are not overcrowded, and do not need relief.
Whereas mixed lines like the above-depicted yellow line are always bad transit, mixed lines like the pink line, in which the circumferential part is farther out than the radial part, are potentially an option for large cities that already have many rapid transit lines. They are especially useful for providing connections between closely parallel radial lines when other crosstown transit options are slow, and should be considered as extensions for relief lines, provided the radial lines farther out do not need relief as well.
Update 2018/2/25: the Jubilee line extension in London is a good example of what I’m describing in this post. The Jubilee line is mostly a northwest-southeast radial line, but to the east of Canary Wharf it heads north to Stratford. The radial alternative would be strong (one of Crossrail’s branches is going to provide it, 20 years after the extension opened), but the circumferential option is even stronger: Canary Wharf is an office cluster and one of the Underground’s busiest stations, and needs rapid transit service from all directions, not just east-west as a pure radial would provide.