Corridor zoning is a style of development, in which major arterial streets are zoned for high residential and commercial density, while side streets remain zoned for low density, sometimes even for single-family housing. Yesterday, a discussion on social media brought up transit-oriented development, so I’d like to explain why corridor zoning is not TOD, in intent or function.

For reference, here’s Yonge Street in Toronto:

And here’s Cambie, in Vancouver:

Both streets have subways running underneath them. Both streets – Yonge far more clearly than Cambie – are characterized by continuous high-density development along the corridor, only extending a block out from it. This is billed as protecting single-family neighborhoods from apartments while also providing opportunities for housing growth; Toronto has fairly YIMBY and Vancouver has very YIMBY housing growth rates, with housing shunted to such corridors and to redevelopable nonresidential sites like Marine Landing. In the United States, corridor zoning can also be found in Seattle, which has nearly the fastest apartment growth rate per capita in the country.

And this is not TOD. The reason is simple: TOD doesn’t care about corridors but about the underlying transit, which is a subway, not a bus stopping every block. From the perspective of a transit user, the walk from the Canada Line 300 meters north-south along Cambie is the same as the walk 300 meters diagonally along Cambie, King Edward, and side streets. Good TOD is therefore measured in terms of walk radius from the station.

Take the following example. TOD zoning might allow a floor area ratio of 3. This is roughly the floor area ratio of the central residential neighborhoods of Berlin, or the inner suburbs of Paris, or much of Upper Manhattan and inner Brooklyn. It can be realized with a six-story building, a common maximum height for North American woodframe construction, using half of a 30 meter deep lot. If these are built 500 meters from a train station, using the taxicab metric, then the resulting 0.5 km^2 area can host around 500,000*0.6*3/45 = 20,000 people at 45 m^2/person with the lots occupying 60% of the area and the streets 40%. This is a cluster that can support a few neighborhood-scale commercial functions, probably at or right next to the station.

The main benefit of corridors is not residential, but commercial. A residential gap in an otherwise commercial corridor will absolutely reduce spontaneity. But continuous commercial development is not achieved through corridor zoning – to the contrary, if the TOD density of FAR 3 can extend sufficiently far from the station, then that density can naturally be developed exclusively away from the corridor, while the corridor itself has the demand for one- and two-story commercial buildings, which can take some residential floors above them in theory but not in practice due to different development needs and different profitability rates. But that’s not corridor zoning – rather, it’s pre-automobile urban development styles, which had isotropic density across an entire neighborhood.

I don’t expect a TOD regime to produce Kreuzberg and Neukölln above. But I do expect it to produce considerable quantities of housing designed around train stations, and if those are modern stations then it’s likely they’re fairly far apart with low-density gaps between high-density pods (but, as much as possible, no residential gaps in commercial corridors). This is not really what’s going on with Canadian corridor zoning, because people can walk in multiple directions, and the explicit purpose of corridor zoning is to concentrate both air pollution and the perceived social pollution of apartment dwellers in quarantine zones that single-family homeowners can avoid. Good TOD practices will therefore drop corridor zoning, and allow much higher density within a radius of several blocks from streets like Yonge, even if it means some single-family homeowners will have to share blocks with mid- and high-rise apartment buildings.