More On: Traffic
Induced demand is the term used by economists to describe how increasing the supply of something (such as roads) makes people want it even more. Though some traffic engineers noticed this phenomena as early as the 1960s, it wasn't until recently that social scientists gathered enough data to illustrate how it happens almost every time we create a new route.
I grew up in Los Angeles, the seaside city beside the freeway. And if there's one thing I've learned since I was old enough to sit up in my car seat, it's that traffic can happen at any time of day. Yes, rush hour is the worst, but I've driven on the 405 at 2 a.m. and encountered dead-stop bumper-to-bumper traffic.
I used to ask my parents why they couldn't just create extra lanes on the highway when I was a youngster. Maybe turn them all into double-decker freeways, with cars whizzing by on both floors. But, as it turns out, that's not going to work. Because if traffic engineers have learned anything in the last few decades, it's that you can't build your way out of a jam. Traffic is caused by the roads themselves.
Induced demand is the term used by economists to describe how increasing the supply of something (such as roads) makes people want it even more. Though some traffic engineers noticed this phenomena as early as the 1960s, it wasn't until recently that social scientists gathered enough data to illustrate how it happens almost every time we create a new route. These findings suggest that the standard methods for reducing traffic congestion are largely ineffective, and that if we could all simply be a little more logical, we'd all be spending a lot less time stuck in traffic.
But, before we get to the remedies, we need to examine the situation more closely. 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 with the total number of miles driven in those cities during the same period in 2009.
"We discovered this ideal one-to-one relationship," Turner added.
Between 1980 and 1990, if a city improved its road capacity by 10%, the amount of driving in that city grew by 10%. If the number of roadways in a certain city increased by 11% between 1990 and 2000, the total number of miles driven increased by 11%. It's as if the two figures were moving in unison, altering at the same precise rate.
Now, correlation does not necessarily imply causality. Perhaps traffic experts in American cities have figured out exactly how many roads to build to meet demand. However, Turner and Duranton believe this is implausible. The contemporary interstate system mostly matches the federal government's 1947 design, and it seems amazingly coincidental that road architects at the time could successfully estimate driving demand more than half a century in the future.
According to Turner and Duranton, a more likely explanation is what they call the fundamental law of road congestion: additional roadways will attract new drivers, resulting in the same level of traffic congestion.
Intuitively, I believe that expanding a road network is similar to replacing a tiny pipe with a larger one, allowing water (or cars) to flow more freely. Instead, it appears that the larger pipe is sucking in more water. The first question that comes to mind is where all these extra drivers are coming from. I mean, do they just appear out of nowhere as engineers construct new roads?
The solution relates to what roads enable people to do: travel around. Humans, it turns out, enjoy moving about. People will travel more if their ability to travel is increased, as they will be forced to drive into town if they live further away from where they work. Making driving easy encourages people to take more road trips than they would otherwise. Finally, companies that rely on roads will swoop into cities with a large number of them, bringing truckers and shipping with them. The difficulty is that all of these factors combine to degrade any extra capacity you've added into your street network, resulting in relatively constant traffic levels. People have an almost limitless urge to use roads as long as driving is simple and inexpensive.
You might imagine that putting more money into public transportation will help to solve the problem. Many railway and bus projects are promoted on this premise, with politicians guaranteeing that as passengers grows, traffic will reduce. However, the statistics revealed that even in cities that increased public transportation, traffic congestion remained unchanged. Some drivers will convert to public transportation if a new subway line is added. However, fresh drivers will take their place. It has the same impact as adding a new lane to a highway: traffic stays the same. (This isn't to suggest that public transportation isn't useful; it allows more people to get about.) According to Turner and Duranton, these initiatives should not be marketed as traffic relievers.)
Surprisingly, the impact also operates in reverse. When a city considers removing lanes from a road, locals shout that it will result in a massive traffic jam. However, the data indicates that nothing absolutely dreadful occurs. The volume of traffic on the route merely adapts, and the total level of congestion does not rise.
For example, in recent decades, Paris has maintained a consistent strategy of drastically shrinking and reducing roadways. "Driving in Paris used to be a nightmare," Duranton added. "It's not much worse, but it's just as horrible."