Wired magazine recently published a good, succinct explanation of induced traffic. It’s worth reading in full as it hits upon an incredibly important, often overlooked fact: it’s not possible to eliminate congestion by building more roads. Here are a few of the more interesting excerpts:
The concept is called induced demand, which is economist-speak for when increasing the supply of something (like roads) makes people want that thing even more. Though some traffic engineers made note of this phenomenon at least as early as the 1960s, it is only in recent years that social scientists have collected enough data to show how this happens pretty much every time we build new roads. These findings imply that the ways we traditionally go about trying to mitigate jams are essentially fruitless, and that we’d all be spending a lot less time in traffic if we could just be a little more rational.
But before we get to the solutions, we have to take a closer look at the problem. In 2009, two economists—Matthew Turner of the University of Toronto and Gilles Duranton of the University of Pennsylvania—decided to compare the amount of new roads and highways built in different U.S. cities between 1980 and 2000, and the total number of miles driven in those cities over the same period.
“We found that there’s this perfect one-to-one relationship,” said Turner.
If a city had increased its road capacity by 10 percent between 1980 and 1990, then the amount of driving in that city went up by 10 percent. If the amount of roads in the same city then went up by 11 percent between 1990 and 2000, the total number of miles driven also went up by 11 percent. It’s like the two figures were moving in perfect lockstep, changing at the same exact rate.
In their excellent paper on the topic, Duranton and Turner describe this as “the fundamental law of road congestion: New roads will create new drivers, resulting in the intensity of traffic staying the same.” Their research also digs into a couple of other related and equally interesting phenomena:
- Better public transport provision doesn’t actually reduce road congestion – but it does enable more people to move without being affected by congestion
- Reducing road capacity has no measurable impact on congestion – if less road space is available, people take public transport or active modes instead, or avoid making low-value trips.
Urbanist.co also has some further discussion of Duranton and Turner’s work. The economists go on to suggest economists’ favourite answer to congestion: road pricing. (If you’re interested in reading more about that topic, Stu Donovan and I have written several posts about the economics of road pricing.)
So what can be done about all this? How could we actually reduce traffic congestion? Turner explained that the way we use roads right now is a bit like the Soviet Union’s method of distributing bread. Under the communist government, goods were given equally to all, with a central authority setting the price for each commodity. Because that price was often far less than what people were willing to pay for that good, comrades would rush to purchase it, forming lines around the block.
The U.S. government is also in the business of providing people with a good they really want: roads. And just like the old Soviets, Uncle Sam is giving this commodity away for next to nothing. Is the solution then to privatize all roads? Not unless you’re living in some libertarian fantasyland. What Turner and Duranton (and many others who’d like to see more rational transportation policy) actually advocate is known as congestion pricing.
Incidentally, I like Turner’s “Soviet Union” metaphor a lot – I’ve said on occasion that we’re running our transport system like a Polish shipyard.
Lastly, it’s incredibly important to consider induced traffic when making policy recommendations. As I wrote in my review of Alain Bertaud’s talks in Auckland, keeping commute times down is an important part of maintaining an efficient urban labour market. Some people seem to have taken Bertaud’s recommendation that policymakers focus on keeping average car commutes under 30 minutes (and PT commutes under 45 minutes) as a call for more roads. This is a superficially appealing but deeply wrongheaded idea.
Induced traffic means that building roads to keep commute times down will not work. And it will be expensive. While there is often a good case for specific road improvements to remove key bottlenecks or improve safety – the Victoria Park Tunnel comes to mind – Duranton and Turner’s work shows that a strategy of building lots of roads will not succeed in minimising commute times. An alternative approach is needed.