Experiment Shows Traffic 'Shock Waves' Cause Jams

Galactic_grub writes "Japanese researchers recently performed the first experimental demonstration of a phenomenon that causes a busy freeway to inexplicably grind to a halt. A team from Nagoya University in Japan had volunteers drive cars around a small circular track and monitored the way 'shockwaves' — caused when one driver brakes — are sent back to other cars, caused jams to occur. Drivers were asked to travel at 30 kmph but small fluctuations soon appeared, eventually causing several vehicles to stop completely. Understanding the phenomenon could help devise ways to avoid the problem. As one researcher comments: 'If they had set up an experiment with robots driving in a perfect circle, flow breakdown would not have occurred.'"

Comments

stability

[backwardMechanic]

'If they had set up an experiment with robots driving in a perfect circle, flow breakdown would not have occurred.'

Is that true? If the robots had been fixed to a set driving speed (open loop), maybe. But if the robots had some sort of collision avoidance, it could still happen. It's instability in the control algorithm, no?

Re:stability

[bondsbw]

Absolutely. Last semester in my graduate robotics class, I had robots follow each other through loops (eventually meant to simulate an intersection control technique). I used collision avoidance on each robot.

I first tested each loop in simulation. The robots would all start at the same acceleration. At a certain saturation of robots, the whole system would break down due to the "waves" of traffic congestion caused by collision avoidance.

Mind you, this was with simulation that was nearly perfect (the only imperfections would have been in thread timings).

Re:Brakes. Not breaks.

[Yold]

Tailgating is a problem too. It really pisses me off, that even in non-rushhour traffic, some idiot is always less than a car-length off my back end. Leaving a buffer zone allows you to avoid using your breaks when traffic slows.

I wonder how much aggressive driving (someone speeding up to 90, and then cutting in front of you for seemingly no reason), contributes to breaking shock waves. I've seen it happen often enough where someone will make an unnecessary maneuver to get 30 feet ahead of traffic.

Re:Interesting, but not a solution

[FuzzyDaddy]

As someone who has been commuting at least 45 minutes for most of my career, I've had a lot of time to think about traffic. I've always thought that traffic can be compared to a phase transition, such as ice freezing. Now this is fuzzy, I haven't done mathematical models or anything.

Breaking or other external factors (an accident or flashing lights by the side of the road) can certainly precipitate a change from a swiftly moving flow to a slow moving flow. However, they only cause a transition when the density is high enough. If there's an accident during a low traffic time, you whiz by it. If they close two lanes out of four, and it's low traffic, you get a little backup, but it reaches a modest steady state size in low traffic. In high traffic you get a "wave" - the backup moves steadily backwards from the scene of an accident, and remains after the accident clears.

I often tell my wife that I can tell if a slowdown is just due to high volume or an accident by the abruptness of the slow down. An abrupt slowdown, I think, means heavy traffic "precipitated" into a jam by an external event.

So braking as described may be a precipitating event, but it's the sensitivity of the traffic flow to it that is the fundamental issue. I'd guess that even if people didn't brake so much, in those sensitive conditions a fender bender by the side of the road could cause a major backup.

(Clearly, I've thought about this WAY too much.)

Speed of light trumps wave speed

[starglider29a]

I have long thought that if there was a pair of LED's in the upper left corner of the vehicle, that indicated "at/above speed limit" or "below speed limit" this would solve many problems. The problem is that, like sound in gas, the notification to slow down is given by the car in front of you only (the molecule about to bump you).

But I could see a half a mile of cars all with little green lights, I could see (at the speed of light) the wave of yellow lights approaching and ease off the gas. The wave would be absorbed by this 'viscosity'. Traffic would flow near the speed limit or average flow rate, whichever the LED's were keyed to.

And don't even get me started on those GPS nav screens. Don't show me were I am. Show me where everyone else is. Let me see the compression 2 miles ahead and I'll chill (heh heh kinetic gas pun).

"Those who forget the past", etc, etc, etc

[Ancient_Hacker]

Very silly article. IIRC traffic researchers in the mid 60's figured out the same thing, by running simulations on a 0.22 MIPS IBM/360. In FORTRAN.

Guys, there really is a benefit to hitting the library and thumbing through back issues of ld technical journals.

Modern Marvels

[Ranger]

I saw a History Channel Modern Marvels episode in highway tech and one researcher was using computer models and he determined it only takes one car to fuck things up for the rest of us. Let me repeat that it only takes one car driving slower than the rest of us to cause congestion and traffic jams on the highway.

Re:That's why I never use my brakes

[T-Bone-T]

When traffic gets slow, I love getting behind a semi. I might not get up to speed as quickly once I'm past the problem area but I can maintain close to the same slow speed for a long time. It makes the situation much less stressful.

Re:Brakes. Not breaks.

[Deadstick]

There's nothing you can do about this.

Maybe not, but there's something your highway authority can do about it: Adopt German rules. Passing on the right gets you a ticket; driving on the left without passing gets you a ticket.

Flipping someone off gets you a ticket too, but that's another issue.

rj

Very cool traffic sim

[nozzo]

I played with this a couple of years back:

http://vwisb7.vkw.tu-dresden.de/~treiber/MicroApplet/

shame this post is buried down deep :-(

Re:Not that simple

[jonbryce]

Maybe you should visit the M42 in Birmingham, England, where a trial of allowing people to use the hard shoulder at peak times was considered a success, and it is going to be rolled out across the rest of the country.

What happens if there is a break down? You are back to three lanes of traffic, just like there would have been if you had never allowed driving on the hard shoulder in the first place. The rest of the time, you get an extra lane.