Fri March 8, 2013
How Closing Roads Can Make Traffic Flow Smoother
It’s no secret that traffic is terrible in Austin. A study from Texas A&M ranked the area 17th worst in the nation – finding the average driver here spent 44 hours of their life stuck in traffic in 2011.
And it probably isn’t going to get better anytime soon.
And when a bunch of downtown streets are shut down for South by Southwest starting today… past experience tells us traffic will be getting even worse.
But does closing roads have to slow everything down? Not necessarily.
First, we need to accept something: When we’re driving, we’re selfish.
Drivers “don’t care kind of the impact of their driving on other drivers, as long as they can get to work as quickly as possible,” said Anna Nagurney. She studies networks at the University of Massachusetts in Amherst. She focuses a lot on road networks.
She’s not saying everyone is a jerk. We just pick the fastest route to get where we need to go. Even when it’s bad for everyone else.
See, you got people in their cars trying to get from point A to point B. Problem is, put enough cars on a road network and it gets all jammed up. What then?
Build more roads? OK. But sometimes that doesn’t work.
“Sometimes, adding roads to a network can actually slow everybody down," said John Hubbard, a math professor at Cornell University.
And he has a little experiment: An imaginary commute that shows how adding a new path can slow things down.
“So you’re in the county of Winding River and there is this river who takes two big loops… so there is a town called Sleepyville on one side – and Factory Town on the other side," Hubbard's experiment goes.
And each morning everyone has to get from their homes in Sleepyville to work in Factory Town. There are two ways to get there. “Either you take first the road around the loop and then a bridge. Or first the bridge and then a road around the other loop," explains Hubbard.
But remember: drivers are selfish. Even the residents of Sleepyville. So each person taking the most selfish route means half the drivers go one way and half the other. “Nobody can gain anything by changing his path. If he decides to change his path it will take him longer to get there,” he adds.
In this experiment, that means it takes everyone an hour and 35 minutes to get to work.
So, what if you wanted to speed things up?
“Now the mayor of Sleepyville and the manager of Factory Town get together and ask Congressman Earmark to build a bridge in the obvious place," Hubbard continues.
So instead of road then bridge or bridge then road, you have a series of three bridges right from Sleepyville to Factory Town. Drivers avoid the roads, skip over the rivers and get to work faster, right?
Well – no.
You see, a curious thing happens here. Everyone is selfish, right? So of course, they’re all going to take what’s now the shortest route. And under the rules set out in this experiment, Hubbard says, all of Sleepytown's commute is now longer.
“In fact, everybody, following his own advantage is now taking two hours and two minutes to get there. The construction of the bridge made everybody slower.”
See, before there were two discreet paths. But the third bridge links those two paths. This is what’s called a Braess Paradox. It’s the theory that sometimes, adding a link in a network makes everything slower.
“I do think it’s much more common than many people realize," said Anna Nagurney. "Because many, many designers don’t realize that this is how travelers behave.”
What’s more, in Hubbard's experiment, no single driver who changes a path, even if the driver goes back to the original path that used to take one hour, 35 minutes, can save any time.
“But actually, the only way to speed up the traffic under these circumstances – is to blow up the new bridge.”
So, can closing down a link in a road network make things faster? The math says: Yes.
“The whole phenomenon is absolutely counter-intuitive," said Hubbard. "How can it be that closing roads can speed up traffic? I mean, after all, there’s less room for the cars to go in. How can it be? But it is.”
Back in the real world, Gordon Derr with the city’s transportation department has to deal with actual roads - not imaginary ones. He says the city has tinkered around with closing streets.
“There’s some streets that we’ve looked at where previously we’ve had four lanes, two lanes of travel in each direction,” Derr said. They cut it down to one lane in each direction. What did they see?
"Well, let’s just say we haven’t turned any of them back, which would be an indication that we really messed up," said Derr.
So not really any effect either way. But also not really a true closure of those roads. But there are actually real world examples of reversing a Braess paradox.
In New York City, on Earth Day 1990. The city decided to close 42nd Street for the day. That’s a major path through Times Square. There were dire predictions for the effect on traffic in Midtown Manhattan.
Seoul, South Korea in 2002, “they had over years built a six lane highway,” said Nagurney. Always jammed, “the highway carried over a hundred sixty thousand cars per day.”
It came down. Result: Better traffic flow.
In 2009, in New York, they closed down Broadway at Times Square, turning it into a pedestrian plaza. Mixed results on that one. Some other streets saw smoother traffic, others saw traffic get worse.
Anna Nagurney says the strategy is a tough sell, despite some successes. But she adds, “if you can make it on Broadway, you can make it anywhere.”
But not in Austin it seems – at least not yet. Gordon Derr from the city's transportation department says they have tried it once.
“When there were questions previously about Riverside Drive through Town Lake Park…we closed it…looked at the traffic was – the impact on other streets," Derr said. In the end, they converted Riverside to a two lane road between South 1st and Lamar, in a tacit admission the idea didn't work in that instance.
But Derr said, if someone has an idea for closing a road, they’ll take a look. And as the number of drivers in Austin keeps on growing, sooner or later, the idea might just be crazy enough to work.