Down's Law of Peak-Hour Traffic Congestion, has been known since 1962; new research shows that it's true even more generally than previously suspected. " data-share-img="" data-share="twitter,facebook,linkedin,reddit,google,mail" data-share-count="false">

Traffic expands to fill the space available. This is known as Down’s Law of Peak-Hour Traffic Congestion, and has been known since 1962; new research shows that it’s true even more generally than previously suspected.

Increasing lane kilometers for one type of road diverts little traffic from other types of road. We find no evidence that the provision of public transportation affects VKT. We conclude that increased provision of roads or public transit is unlikely to relieve congestion.

Eric Jaffe draws a simple conclusion from all this:

Whenever a driver shifts onto public transportation, another one quickly grabs the open lane. That leaves just one solution to the traffic problem plaguing American cities: congestion pricing. “We cannot think of any other solution,” says Gilles Duranton, the paper’s co-author. “As soon as you manage to create space on the road, by whatever means, people are going to use that space. Except when people have to pay for it, of course.”

I’m a fan of congestion pricing. But I’m also realistic about it, and the fact is that for all Jaffe’s enthusiasm, congestion pricing has its own Down’s-like characteristics. Jaffe raves about the “success” of congestion pricing in London and Stockholm, but the only chart he provides gives numbers for the trial period in Stockholm. If you go looking for recent data on traffic in London, Stockholm, or other cities with congestion charges, it can prove surprisingly difficult to find.

Here’s Jaffe, again, quoting Duranton:

“My feeling is, yes, people tend to be against it before they see it at work,” says Duranton. “They think it’s going to cost them more money, which directly it will, but they’re all very unclear about the benefits; i.e. traffic is way more fluid, way faster, and pollution is going down.”

There’s another way to look at this phenomenon, though. When congestion pricing is first introduced, people recoil against it — they expend quite a lot of effort to avoid the charge, and traffic goes down. Over time, however, it becomes just another part of the cost of driving, along with gas and insurance and parking tickets. As that happens, traffic goes back up again. Congestion-charge revenues go up too, of course, and those can be reinvested into public transport.

But traffic is like water — it wants to find its own level, which tends, in cities, to be maximum capacity. If you want to implement a system which keeps traffic below maximum capacity, then you need to apply significant pressure on drivers to keep them away from the roads. And that means not just implementing a congestion charge, but also regularly increasing the amount of the charge over time.

This is how the Singaporean congestion-charging system works. Think of a shutter-priority camera: you set the shutter speed, and then dial the aperture so that the exposure is correct. In Singapore, they set the amount of traffic they want, and then dial up the congestion charge until they get it. It’s much the same idea as the one behind SFPark: you set the number of empty parking spaces you want, and then dial up the parking-meter pricing until you get there.

But the point in all of these cases is that the charge has to be variable over time — specifically, it has to increase over time. Without those steady increases, drivers become inured to the congestion charge, and traffic will go back up to its former level.

As a result, drivers are pretty much never happy with congestion pricing. Either it’s painfully expensive and going up in price — expensive enough to keep them from driving — or else it doesn’t have much effect.

That doesn’t mean that congestion pricing isn’t good public policy. It is. But it’s always going to be unpopular with a powerful constituency. (Drivers, in nearly any city you care to mention, tend to have a disproportional amount of political clout.) Local politicians looking for a popular platform will run on reducing or abolishing the charge, or at the very least not increasing it. And so the old fight keeps on being fought over and over again: while increasing a charge isn’t as politically difficult as introducing one, it’s still tough.

This is something worth remembering when urbanists start waxing utopian on the subject of congestion pricing: once it’s introduced, the fight isn’t over. It’s never over. And if you leave a system long enough without increasing its price, its efficacy starts declining dramatically.