A while ago, I pretty much lost a debate in the comments with transit writer extraordinaire Alon Levy. At issue was whether mode share – the percentage of commuters who use transit, cars, biking, etc., to get to work – was the best way to measure the effectiveness of a city’s public transportation. The debate was provoked by yet another ranking of cities by transit mode share, and my discomfort with the triumphant reaction from some quarters.

The argument in favor of mode share is basically that people will do whatever is easiest and most convenient; if very few people are using transit, that’s a pretty good indication that it isn’t easy or convenient for the vast majority of people.

My counter-argument was that mode share measures relative ease and convenience: if 10% of people take public transit, that tells you that transit is more convenient than driving for roughly 10% of commuters, but it doesn’t tell you if, for the other 90%, transit service is perfectly acceptable, but driving is just easier; or if transit service is actually terrible.

I think, though, that the new report on jobs accessibility from the University of Minnesota should reopen the debate for at least as long as it takes for Alon to convince me again that I’m wrong. In particular, Jacob Anbinder’s interpretation of the study at Real Clear Politics, which looks at the percentage of regional jobs accessible by public transit within an hour (the original report focused on raw numbers). And, when you do that, it becomes clear that the massive, older regions with very high transit commute share – New York, Chicago, DC, etc. – have a problem: namely, that getting to jobs outside their dense cores is very, very difficult. At the same time, smaller regions with little to no reputation for public transit – Salt Lake City, Milwaukee, Denver – look much, much better.

There are a number of things at work here, I think, which all boil down again to the fact that mode share measures relative convenience, not absolute. In the dense centers of America’s largest, older cities, transit is relatively convenient, and driving is relatively inconvenient for reasons of traffic, parking, etc. But as you leave the center, transit becomes dramatically less convenient – especially if you’re not going to the center, which is often served by commuter rail lines – and driving becomes dramatically more so. What that means is that for a relatively large number of people who live in the core, transit is the better option, leading to high mode shares; but outside the core, transit service isn’t just less convenient than driving, it’s actually pretty bad.

In some smaller metropolitan areas, though, transit may be so-so over a much larger percentage of the region, while driving is basically always convenient. These cities lack the cores that give places like Boston a high mode share, but provide decent enough transit service over a large enough portion of the region that if you really want to use transit – because you want to save money, or you’re too old/young, or you just don’t like driving – it’s not a disaster.

Here is this idea in graph form:

That’s how you arrive at the fact that the average commuter can get to 25% of jobs in metropolitan Salt Lake City within an hour, but only 15% of New York’s.

Now, there are a number of caveats to this. In particular, using total commute time as a cutoff (jobs accessible in an hour) inherently disadvantages geographically large metropolitan areas like New York or Chicago. First, obviously, because the larger the area, the longer it takes to get places, all else equal. But also, as a region expands, the challenge of making any given point-to-point trip doable on transit requires much, much larger investments in infrastructure: while a smaller region might be able to get away with a well-organized bus system, and a medium-sized region with buses and a few radial rail or BRT lines, a region the size of Chicago or LA needs an extensive bus system, radial rail routes, and some kind of cross-town rapid transit service.

More significantly, as a good leftist, I believe that inequality matters. It may be that in Salt Lake City, a 60 minute commute on public transit would take only 20 minutes by car, while in New York, a 60 minute transit commute would be 45 minutes by car. In a region where the expectation is that commutes will be relatively short, transit accessibility that requires dramatically longer trips will probably relegate transit to a sort of welfare service, patronized only by those who absolutely, absolutely have to. That kind of stigma is a problem for all sorts of reasons: to begin with, I’m against stigmatizing people; but also, from a political perspective, it probably makes it harder to lobby for improvements to public transit if it has the reputation as a service exclusively for poor people.

Still, I think this report adds some strength to the idea that mode share is a very incomplete look at a region’s transit effectiveness. Also, happily, the widespread attention it’s received is a pushback against the kind of urbanist lifestyle fetishism that frequently accompanies rankings based on mode share. (See, for example, the story I quote at the beginning of my earlier post.) A functional transit system ought to be about making day-to-day life easier for regular people, and focusing on job access highlights that. Yay.