Monday, July 15, 2013

The Fundamental Law of Highway Congestion

The first topic I'd like to address on the blog is congestion. This is something we're quite familiar with in the Seattle area. Not a day goes by without a traffic backup somewhere. We are, after all, a city of chokepoints. There are only two north-south highways leading into downtown Seattle, I-5 and highway 99. Likewise, there are only two east-west highways leading in as well: state route 520 and I-90, both of which have bridges to cross Lake Washington.

Common sense dictates that if you don't have enough lanes of traffic to handle the number of vehicles passing through, all you need to do is increase the number of lanes of traffic. It's true that this does increase your capacity. But does it reduce congestion?

A 2009 study on the subject suggests that it doesn't, in a paper by researchers Gilles Duranton and Matthew A. Turner. They cite something called the Fundamental Law of Highway Congestion first suggested by Anthony Downs in 1962. Downs himself said the following (cited from this source):

[E]xpansion of road capacity – no matter how large, within the limits of feasibility –
cannot fully eliminate periods of crawling along on expressways at frustratingly low speeds.

Basically, there is only a certain amount of congestion that people are willing to tolerate. Where alternative modes of transportation exist - buses, rail, bicycle paths, and so forth - people will use those modes of transportation to avoid congestion. But the latent demand is still there. When road capacity is expanded, that latent demand fills up the new capacity rather quickly.

The study offers only one solution for congestion that is proven to be effective, and if you're a driver, you're not going to like it: congestion tolling. By charging a higher toll on highways based on when the most demand is there, people naturally find alternatives and congestion drops.

It's also worth noting that the first paper cited finds no relationship between the increase in the availability of public transportation and reducing congestion. This is interesting because reducing congestion is often used as a justification for creating transit. This all goes back to the fundamental law: if people start using transit and leave their cars at home, the latent demand for highway lanes will manifest itself and the congestion will keep at the same level it was at before. Ultimately, you are moving more people with transit than you are without. There are better reasons for building transit than claiming it will reduce congestion - that is probably the least important reason, even if it's true.

Here's a couple other articles that cite this study, for your reading pleasure:


The Atlantic

I'll revisit the topic of congestion, congestion pricing and the fundamental law in the near future. I'm hoping to go into more detail and look at a few case studies. In the meantime, the next big topic I will begin to cover will be where the funds for transit systems (buses in particular) are coming from.

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