A while ago, someone asked me what we could do to solve traffic problems. I was feeling flippant, so I said: “Birth control.”

I was reminded of this in a recent flurry of articles about overpopulation (here, for example), something that was broached again with the sordidly surreal “octuplets” case.

But in the U.S., at least, not all the news is bad, as reported by the New York Times:

“And besides, they say, the birth rate in the United States is barely at the level needed to replace the population. Total fertility rate, which predicts the number of children an average woman will have in her lifetime, reached 2.1, considered replacement level, in 2006, but it was the first time it had been that high since 1971. A small percentage of large families, they say, is not enough to tip the balance.”

Given that we’ve added roughly 100 million people since 1968, one does have to wonder, however, about the link between a growing population (via births or immigration) and consistently rising travel times (NB: traffic is far down on my list of “things to be worried about by a growing population”). But looking at some research by Steven Polzin, at the University of South Florida, this relationship is not as simple as it might seem.

The first thing to note, as in the slide below, is that while population has been on the rise, it is far outpaced by “vehicle miles traveled.” It’s not just how many people we have, it’s how much they’re driving.

And while it is inherently true that larger families consume more resources, including miles traveled, there is something of an “economy of travel” as household size increases. Most interesting, though, for travel demographers is the increasing number of single-person households, as in the slide below.

Also of interest is the issue of rising income, and the decreasing cost of travel.

And there’s the people who didn’t have cars who increasingly seem to have one.

Polzin himself does not put population at the top of causal factors for the increase in VMT (and thus, relatedly, congestion); but instead, “trip frequency.”

It goes without saying that population plays some role in contributing to congestion. But what is arguably more critical is a number of demographic changes in the population itself, including the way it has chosen to live (non-walkable neighborhoods in far-flung suburbs, meaning more trips and longer trips — a trend that Richard Florida suggests may have reached its zenith in the current economic meltdown), declining walk-to-work and walk-to-school shares, increasing numbers of cars per household, higher shares of licensed teenage drivers, declining carpool participation rates, under-priced car travel, declining transit funding, etc. etc.

For an example of how things can be done differently, we can look no further than NYC, where a recent study found that between 2003 and 2007, even as the city continued to grow (economically and population-wise), vehicle traffic actually dropped. Granted, the subway cars have felt more crowded since the introduction of the Metrocard, but unlike vehicular traffic, an absolutely packed train travels at essentially the same speed as an empty one.

This entry was posted on Monday, February 16th, 2009 at 10:36 am and is filed under Congestion, Etc.. You can follow any responses to this entry through the RSS 2.0 feed. Both comments and pings are currently closed.