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Froggie: The next part will be addressing your point about increasing capacity.

Jamie: It's not a net zero sum game at all. That's the classic traffic engineer assumption and it's false.

A lot of people make decisions about how to commute every day. A five minute swing will shift some people from Metro to driving. We don't really know how many, but with hundreds of thousands of people commuting, it's not crazy to think maybe a couple percent would shift.

Also, some people telecommute. Some people choose to carpool. Our expansion of freeways always affects these decisions.

Also, people choose where to live based on the transportation options. This is the fundamental driver of induced demand. The research is voluminous. When cities built freeways throughout the 20th century, predicting that a set volume of traffic needed a wider freeway, they instead found that within a few years of the wider freeway, it filled up again. That's because, when the wider freeway came online, people decided they could now live in new, farther-out areas and still get to work in the same amount of time as before.

Traffic does pollute more moving slowly, but it pollutes a lot more by existing in the first place. A fast-moving yet full eight-lane freeway generates a lot more pollution than a slowly-moving four-lane local road. And a traffic-choked eight-lane freeway, which the fast-moving eight-lane freeway eventually turns into, is even worse yet. Again, you assume that there are a set number of people who will drive from set of points A to set of points B, and it's just a network optimization problem. That's not how traffic works in reality because people base whether to commute, how to commute, and where to live based on the options available.

You're right about the summary of my argument. We shouldn't add more freeway capacity (not "improve"; adding more isn't necessarily an "improvement") because it will indeed encourage people to drive more. We have plenty of evidence of that from seventy years of adding roadway capacity all over the nation.

We shouldn't neglect the road infrastructure, but neither should we spend a lot of money (and this is a huge amount) adding a lot more of it.

by David Alpert on Feb 11, 2009 4:59 pm • link • report