Traffic: The (Almost) Unsolvable Problem
We’ve all been stuck on a highway in bumper to bumper traffic before. While we’re sitting there, going nowhere, it’s easy to imagine how much less futile our current state of existence would be if only there were another lane or two where the shoulder currently sits. There just isn’t enough roadway space, and if we added some more, the traffic would break up and we’d be on our way. Right?
This is the logic departments of transportation around the country have used to plan our highways since the invention of cars. Engineers today have come up with more precise formulas to measure traffic levels, like the “volume to capacity” ratio, but the basic logic has remained the same. Traffic flows, like a liquid, so we need to make sure the container is large enough to prevent bottlenecks.
Unfortunately, that logic is wrong. Traffic isn’t like a liquid. It’s like a gas. It expands to fill its container. That’s why a century after we started expanding highways to make room for cars, they’re more congested than ever.
There are two reasons traffic is more like a gas than a liquid. One plays out in the short-term and the other plays out in the long-term. The short term reason goes by the name “convergence”. Here’s how it works.
Right now, most roads in the U.S. don’t cost money to drive on. However, that doesn’t mean they’re free. Traffic takes up drivers’ time and gas, so congested roads do impose a cost on them. Because drivers want to avoid that cost, some of them take other routes, travel before or after rush hour, or take the train.
When engineers widen a road, congestion (temporarily) eases. In other words, the cost to use it goes down. Responding to this drop in price, some of the drivers who had previously avoided the road converge back onto it, using up the new capacity and bringing the bumper to bumper traffic back with them.
We can find examples of this phenomenon (and of transportation engineers ignoring it to their own peril) just about everywhere. It’s fun to pick on Texas, though, so I will. The Texas Department of Transportation spent almost three billion dollars to expand Interstate 10 in suburban Houston from 8 lanes to 23 (!) lanes. The result? After some initial improvement, traffic conditions quickly eroded to their typical levels. That’s convergence in action.
The long-term reason that highway widenings don’t work is called “induced demand”. To illustrate this one, I’ll use an example from San Antonio. The Texas Department of Transportation is spending over a billion dollars to expand a ring road 15 miles from the city center from four to ten lanes. They say the expansion is necessary. They’re expecting a lot of population growth in the area, and this growth will lead to a lot of cars that need a lot of road space. Specifically, they expect traffic volumes to double over the next 20 years.
But here’s the flaw in their thinking. That growth doesn’t happen there on its own. The kind of sprawling, suburban growth they’re projecting crops up only where highways make car travel the most logical choice. One of the themes of this blog is that our transportation infrastructure greatly influences our land use patterns. If we build transportation systems that make it easier for people to walk, bike, and take transit, more people live near their daily needs and use those modes. If we build transportation systems that make cars the only logical way to get around, more people live further from their daily needs and drive pretty much everywhere they go. That’s what we’re seeing in San Antonio. By building more highway infrastructure, engineers “induce” the very traffic problems they’re trying to solve.
If we want to cure congestion, trying to build our way out of it isn’t going to work. To get to the heart of the problem, we don’t need wider roads. We need a transportation system that provides better travel options. If most people can walk or take the bus to work, shopping, or school, the next billion dollar highway project won’t be necessary. We also need a price to use the road that doesn’t decrease when congestion eases. That way, people won’t come spilling back on to the road when it starts working again. That’s the principle behind congestion pricing, which I’m a big fan of and will talk more about in the future. For now, though, I’ll leave it at this: widening highways doesn’t cure congestion. It just puts more cars on the road.
And as I’ll talk about a lot on this blog, putting more cars on the road is a very bad thing. It’s bad for (deep breath) climate change, air quality, car crashes, public health, social interactions, urban design, people who can’t afford a car, our reliance on foreign oil, and the economy as a whole. But those are all topics for another day.
