Past studies, including one published in 1977 by Boris S. Pushkarev and Jeffery M. Zupan, have shown that public transit works best where gross residential densities are above 4,200 persons per square mile; relatively dense housing is clustered close to transit stations or stops; and large numbers of jobs are concentrated in relatively compact business districts.

I actually have the Public Transportation and Land use Policy right in front of me. And in several sections, Pushkarev and Zupan explicitly stress that gross density is a “very poor estimator of residential density”, and that they don’t use it in their comparisons of cities.

So that pretty much destroys a huge chunk of Mr. (Dr.?) Down’s argument right there.

Every large urbanized area is required to have a Metropolitan Planning Organization (MPO) that carries out transportation planning, forecasting, and allocation of federal funds. MPOs use geographic units for transportation planning that are called traffic analysis zones (TAZ). TAZs are drawn using streets and roads as boundaries. They are drawn to have roughly equal populations, so in dense areas they will be small (such as a few blocks) and in rural areas they will be large (such as thousands of acres).

One very useful feature of TAZs is that they include both the residential and work populations. You can see how this leads to some unexpected results in The Density of Traditional Urbanism.

Roadway planning is relatively uncomplicated. A certain number of residents and jobs are forecast for a TAZ; these generate a certain number of trips; and therefore a certain amount of roadway capacity is required. That’s still the conventional theory, although it has been seriously debated in recent years.

Transit planning is much less standardized, because transit modes are more diverse and transit ridership is more complex to model. Unlike roadway planning, transit planning has to consider things like the quality of walking routes, feelings of safety, and other hard to quantify issues. A good starting point for learning more might be the report Fixed-Route Transit Ridership Forecasting and Service Planning Methods.

Even in the U.S. the quarter-mile rule of thumb is too short in some contexts. Recent research by Schlossberg et al. found the mean distance pedestrians will walk to a light rail station is half a mile, especially if the route is convenient, safe and pleasant.