This story has been receiving a lot of buzz:
Last year, the US made more progress in reducing traffic congestion than any other time in memory. New data show that in the nation’s cities congestion declined by 30 percent overall and was improved at every hour of the day.
How did we make these big gains? Not by adding more highway lanes or transit. Our physical infrastructure barely changed. Rather, we did it with a very modest decline in car travel. On urban interstate highways, total vehicle miles traveled in the US declined by about 3 percent compared with 2007.
The dramatic decline in congestion — which analysts have labeled “startling” – was almost universal. It actually fell in 99 of the nation’s 100 largest metro areas, according to Inrix, which monitors traffic around the nation. The company’s data come from tens of billions of reports from GPS-equipped vehicles traveling the nation’s roads, the same data that provides real-time traffic information to commercial users and web-services like Mapquest, Garmin and On-Star.
Their key conclusions: “peak hour congestion on the major roads in urban America decreased nearly 30% in 2008 versus 2007*,” and nationally, “congestion was lower every hour of every day in 2008 versus 2007 – between 15% and 60% lower depending on the hour and day.” (See the full report here.)
How did such a small decline in travel produce such a big drop in congestion? It’s well known that traffic congestion is subject to a tipping point–what economists call non-linearities. Add an additional car to a crowded road at rush hour, and traffic slows down a bit, and then the “carrying capacity” of the road declines. Traffic engineers estimate that most roads carry their maximum throughput — number of vehicles per hour at about 40 miles per hour — so as traffic slows below that speed, the road actually loses capacity and goes slower and slower, producing a traffic jam.
But the same is true in reverse. Take a few cars off the road at rush hour, and traffic moves faster, and highways can actually carry more vehicles. And in every large American city, that’s exactly what has happened in the past year.
As the Inrix study concludes, “When a road network is at capacity, adding or subtracting even a single vehicle has disproportionate effects for the network. This phenomenon has been well known for a long time, but this data illustrates it in real-world terms on a nationwide basis.”
We should apply this lesson to hypercongested I-35. While I've suggested before that it might take high variable prices to make a difference on I-35, perhaps that's not true. Perhaps even small tolls could take eough cars off I-35 to get traffic flowing.
We should start with the big rigs passing through to points north or south of town. A tractor-trailer creates much more congestion than a passenger car. It occupies twice as much space and starts and stops sluggishly. It also does several times as much road damage, and damage to a heavily-traveled road is much more costly than damage to a lightly-traveled road. By charging these dinosaurs high tolls, we would encourage them to circumvent Austin via SH 130.
Modest tolls for passenger cars might get us the rest of the way home. A $1 toll for the trip from, say, Pflugerville or Slaughter to downtown Austin might take just enough passenger cars off the road -- perhaps just 1 or 2% -- to speed up the average traffic flow. Charging suburban drivers $10/week might shave an hour or two off their weekly commutes. Surely most drivers would think that was a good deal, no?
