This is my third post about scale variance in transit planning; see parts 1 and 2. In part 1, I discussed how good bus networks exist at a certain scale, which can’t easily be replicated at larger scale (where the slowness of city buses makes them less useful). In part 2, I went over a subway planning feature, especially common in the communist bloc, that again works only at a specific scale, namely cities with enough population for 3-4 subway lines; it gets more complex in larger cities, and cannot be imported to bus networks with 3-4 lines. In this post, I will focus on one scale-variant feature of surface transit: the grid.

The grid works only for surface transit and not for rapid transit, and only at a specific scale, so constrained as to never be maximally useful in an entire city, only in a section of a city. This contrasts with what Jarrett Walker claims about grids. Per Jarrett, grids are the perfect form of a transit network and are for the most part scale-invariant (except in very small networks). One of the impetuses for this post is to push back against this: grids are the most useful at the scale of part of a transit city.

Grid Networks Versus Radial Networks

I’ve written a few posts exhorting subway planners to build their networks in a certain way, which, in the most perfect form, is radial. In particular, tangential subway lines, such as the G train in New York (especially when it ran to Forest Hills), Line 10 in Paris, and Lines 3 and 6 in Shanghai, are weak. When the G train was running to Forest Hills, most local passengers would switch from it to the next Manhattan-bound train, leading New York City Transit to send more Manhattan-bound local subways to Forest Hills and eventually to cut back the G to Long Island City. Based on these examples, I contend that on a subway network, every line should be either radial, serving the CBD, or circumferential, going around the CBD.

My post about New York light rail proposes a network with some lines that are neither: in the Bronx, my proposal is essentially a grid, with north-south routes (Grand Concourse, Webster, 3rd) and east-west ones (161st, Tremont, Fordham) and one that combines both (145th-Southern). Regular commenter NewtonMARunner criticized me for this on Twitter. I answered that the lines in my proposal are based on the busiest buses in the Bronx, but this simply shifts the locus of the question to the existing network: if transit lines should be radial or circumferential, then why are the tangential Bx19 bus (145th-Southern) or the Bx40/42 and Bx36 (Tremont, with a long radial eastern tail) so successful?

To answer this requires thinking more carefully about the role of circumferential routes, which by definition don’t serve the most intensely-used nodes. In Paris, Lines 2 and 6 form a ring that misses five out of six train stations and passes just outside the CBD, and yet they are both busy lines, ranking fourth and fifth in ridership per km. The reason is that they are useful for connecting to radial Metro lines and to some RER lines (namely, the RER A and the southern half of the RER B). Tangential lines miss connections much more easily: in the west, Line 10 here has a decent transfer to Line 9 and a somewhat decent one to Line 8, but to Lines 12 and 13 it’s already not very direct. The G train in New York has the same problem to the south – few connections to lines that actually do go into Manhattan.

Consider the following three possible networks:

The radial network is a typical subway network. The full grid lets you go from everywhere to everywhere with just one transfer, at the cost of having far more route length than the radial network. The partial grid no longer lets you go from everywhere to everywhere easily, and has the outer two lines in each service direction missing city center, but still has more overall route-length than the radial network. The principle here is that a grid plan is useful only if the grid can be complete.

The scale, then, is that rapid transit is so expensive that there’s no money for a complete grid, making a radial plan more appropriate. But surface transit, especially by bus, can be spread across a grid more readily. The Bronx’s size, density, and bus ridership patterns are such that a mostly complete grid is feasible within the western two-thirds of the borough, supplemented by the subway. In this environment, a tangential route is fine because it hits all the radial routes it could, and could provide useful two-seat rides to a large variety of destinations.

Are Grids Really Grids?

Chicago has a relentless bus grid. The three busiest north-south routes are the tangential 8 (Halsted), 9 (Ashland), and 49 (Western), which are 22, 29, and 26 km long respectively. None enters the Loop; Halsted, the easternmost, is at the closest approach 800 meters from the Loop, across a freeway. The two busiest east-west routes, the 77 (Belmont) and 79 (79th), are also far from the Loop.

However, I contend that these routes don’t really form a grid, at least not in the sense that passengers ride between two arbitrary points in Chicago by riding a north-south bus and connecting to an east-west bus. Instead, their outer ends form tails, which people ride to the L, while their inner ends are standard circumferentials, linking two L branches. The L in turn is purely radial and doesn’t follow the Chicago grid, with the Blue Line’s O’Hare Branch, the Orange Line, and the Brown Line all running diagonally.

Vancouver is similar. The north-south routes are radial, veering to enter Downtown. The east-west ones are more circumferential than tangential: they connect the Expo and Canada Lines, and most also connect to UBC. The Broadway buses (9 and 99) pass so close to Downtown Vancouver they’re more tangential, but they also offer the shortest path between the Expo and Canada Lines (making them a strong circumferential) while at the same time serving high job density on Central Broadway (giving them some characteristics of a radial).

In the absence of a radial rail network to connect to, long grid routes are less useful. Cities have a center and a periphery, and the center will always get more ridership, especially transit ridership. The outermost grid routes are often so weak that they should be pruned, but then they weaken the lines they connect to, making it necessary to prune even more lines until the grid is broken.

The Optimal Scale for a Grid

A strong transit grid will not form in a city too small for it. There needs to be a large enough center with enough demand for transit ridership to justify more than a purely radial bus network with a timed transfer. At the same time, the city cannot be too big, or else the arterial buses are too slow to be useful for ordinary work and leisure trips, as in Los Angeles.

What’s more, there is no Goldilocks zone, just right for a grid. Chicago is already too big for a bus grid without the radial rail layer. It’s also too big for what Jarrett calls grid accelerators – that is, rapid transit routes that replace bus grid lines: the Red Line is plausibly a grid accelerator, but the other lines in Chicago are not, and if there were L lines only at grid points, then the Red Line and the one east-west route would get overcrowded heading toward the Loop. Even Vancouver, a compact metro area hemmed by mountains and the ocean, relies on the diagonal Expo Line to serve Downtown and doesn’t really have a grid beyond city limits. A less dense city in the same land area could have a grid, but without much traffic or a strong CBD, cars would always beat transit on time and only the poor would ride the bus.

The scale in which grid networks work more or less on their own seems to be that of Vancouver proper, or that of the Bronx. Vancouver is 115 km^2 and the Bronx is 110 km^2; Vancouver’s bus grid spills over to Metrotown and the Bronx’s to Upper Manhattan, but in both cases these are small increases in the relevant land area.

Tellingly, Vancouver still relies on the bus network to feed SkyTrain; the Canada Line is a grid accelerator, but the Expo Line is not. The Bronx is the more interesting case, because it is not a city or even the center of the city, but rather a dense outlying portion of the city with an internal arterial grid. In both cases, the grid is supplementary to the radial rail core, even if the routes that use it have a lot of independent utility (Metro Vancouver has higher bus ridership than rail ridership, and the Bronx buses combined have slightly more ridership than the combined number of boardings on the Bronx subway stations).

Geographical constraints matter as well. The Bronx and Upper Manhattan are hemmed by water and by the administrative border of the city (which also includes a sharp density gradient), and Vancouver is hemmed by water and by a density gradient in the east. This makes it easier to equip both with grids that are close enough to the complete grid in the middle image above rather than the incomplete one in the third image. The Bronx’s lower-density eastern tails happen to meet up with those of Queens, forming circumferential routes, and also have enough north-south subway lines to feed that they remain useful.

In a transit city, the grid cannot come first. Even if there is a street grid, the spine of the network has to be radial as soon as there is demand for more than two rapid transit lines. The role of surface transit remains feeding rapid transit. Grids look attractive, but the optimal scale for them is awkward: large-scale surface transit grids are too slow, forcing the city to have a rapid transit backbone, and if the city is too small for that then the arterial grid provides too good auto access for public transit to be useful.