One of the first posts I ever wrote for City Observatory was called “Undercounting the transit constituency,” and it made a simple point: We dramatically undercount the number of people who depend on public transit to get around.

While we usually talk about transit use in terms of the number of people who ride a bus or train to get to work, most trips aren’t commutes—in fact, all trips aren’t commutes if you’re retired or a full-time student. And so if you count the number of people who use transit for any kind of trip, you get a much bigger number than if you just look at commutes.

But even when it comes to workers, we often overstate the ubiquity of cars. One common way to measure car access, for example, is the percentage of workers with “access to a car.” In practice, “access” just means that someone in your household owns a vehicle.

But that kind of “access” doesn’t always mean much. For example, for several years I lived in a household with three working adults, one of whom owned a car. By the standard measure, all three of us had “access.” But in reality, only one of us could actually use it to commute. Moreover, even the one of us who drove to work really depended, in a meaningful sense, on public transit: if the other two of us hadn’t been able to get to work, get paid, and contribute our share of the rent, the one driver would still be in deep trouble. In other words, a household with at least one worker who doesn’t drive is a household that probably depends on some kind of non-car transportation.

How many workers live in such a household? Fortunately, the American Community Survey makes it easy to tell, by breaking down the number of cars in a household by the number of workers in that household. Here, then, is a chart with the proportion of workers without “access,” as well as the proportion of workers in households that have fewer cars than workers:

In most cases, these numbers are quite different. In the DC metro area, less than six percent of workers live in zero-car households—that is, “lack access to a car.” But three times as many—nearly one in five—live in a household with fewer cars than workers. In San Diego, “access” is nearly universal: just 2.6 percent of workers live in zero-car households. But more than one in ten live in fewer-cars-than-workers households.

And again, these numbers underestimate the number of people who live in households that rely on some kind of non-driving transportation. They don’t count households where each worker has a car, but a retiree, stay-at-home parent, or full-time student, for example.

But even leaving that aside, these numbers help make a better case on how your city’s workforce depends on non-car transportation.