Traffic congestion sometimes seems like an utterly in­trac­table problem. If a city suffering from gridlock tries to construct new roads to ease the pressure, traffic usually just increases to fill the extra space. Same thing when new bus and subway lines get built. Yet a few cities like Stockholm, London, and Singapore have taken a more market-oriented approach to congestion — treating it as an externality and essentially taxing it the way one might tax pollution.

And it seems to work. Over at Atlantic Cities, Eric Jaffe breaks down a new study from three Swedish researchers finding that congestion pricing in Stockholm has created a sustained drop in traffic over the past four years:

The key word here is “sustained.” Earlier critics of congestion pricing worried that drivers would eventually get used to the tolls (Singapore, for one, adjusts its tolls to get the desired level of traffic flow, which sometimes means cranking prices up to painful levels). But Stockholm puts a flat $2.60 charge on all vehicles entering the city during peak hours and a $1.30 to $2 levy at other times throughout the day. And, so far, traffic appears to have stayed at reduced levels since the system came into effect in 2007 (the charge was passed into law after an earlier trial period in 2006).

About 20 percent of the drop in Stockholm came about because Swedes shifted to public transit. Meanwhile, commercial traffic into the city — deliveries and freight — dropped by about 15 percent as companies adjusted to the new charges by switching routes or combining trips. Many of the feared adverse effects, like excessive strains on roads not covered by the charge, didn’t seem to pan out. Moreover, the policy appears to have boosted sales of ethanol-powered cars and fuel-efficient vehicles, which get an exemption from the fees.

The researchers point out that the new policy seems to be quite popular, polling at about 70 percent in May of this year, although it’s not clear exactly why. Perhaps Stockholm residents enjoy having fewer traffic jams and delays. Perhaps people didn’t seem to mind adjusting their driving patterns or shifting over to transit. It’s worth noting that Stockholm has a pretty good public transportation system, which makes it less painful for people to adjust to congetion pricing.

Congestion charges have had a hard time getting traction in the United States — the New York state government famously killed a proposal from Michael Bloomberg to put a congestion charge on Manhattan south of 60th Street. There are worries that it would be overly intrusive, or that it would unfairly burden the poor. One lesson from Stockholm seems to be that most of these complaints dissipate pretty quickly once the system’s in place — provided that congestion actually clears up.