You know those strange traffic jams that appear to come out of nowhere, with nothing causing them, and then suddenly end? As Wired reports, a team of M.I.T. mathematicians calls them “phantom jams” or “jamitons,” and has found mathematical equations to describe them, similar to those that describe detonation waves from explosions. Phantom jams, the mathematicians found, can form when a single driver slows down (to take a sip of coffee or talk on the phone) on a road with too many cars on it. They hope the new equations will lead to roads engineered to keep traffic below the density where a jamiton can form. [%comments]
Ghost Jams
TAGS: traffic
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I did a paper on these in an undergrad class like 20 years ago; don’t need any fancy math to figure out what’s going on. I called these jams “Sheep Blocks”. The legend is that if you get a herd of sheep to start jumping over a short barrier, then pull the barrier out when the herd is half way over it, the later sheep will continue to jump over the same spot even though the barrier is gone.
As your MIT guys figured out, in dense traffic, if a single car slows down, it forces other cars to slow too and if the density is high enough the compressed traffic remains long afterwards.
More interestingly, the “sheep block” itself can migrate up stream if traffic density remains high (analogous to sound).
It is also an interesting trick used in Indy Car and NASCAR racing on a restarts. The lead driver slows just a bit coming out of curve 3 causing the entire pack to slow down. Exiting from curve 4 down the straightaway allows the leader to get an extended jump on the rest of the pack still recovering from the slowing “phantomjam”. A good track official will spot the signs and force another yellow lap on the field.
Gotta love IRL and NASCAR to find another way to win!
Out here in California when vehicles are speeding or there is a possible road hazard ahead, the CHP gets in front of the pack and does a “snake dance” to slow down traffic. A single CHP vehicle swings from lane to lane to force everyone to slow down. I guess, the opposite is true when you are driving within the speed limit and everyone around you speed up and you are forced to keep up with the pack otherwise you can get hit. I could observe this happening usually at the beginning and end of long weekends when everyone is trying to head in one direction.
We’ve always called these slow down’s “slinky’s”, due to their variations in speed.
I’m sure the math is interesting, but the conclusion is somewhat obvious.
I am not sure what they would have found.
I remember reading in Scientific American 40 years ago of equations for the waves or pulses or clusters – choose your term – of vehicles on freeways.
That article was somewhat oriented to the persistence of clusters and their movement down the freeway. I don’t think it said much about why jams first form.
It is established that roadways have a maximum flow after which average speed will fall rapidly. That is why metered on-ramps are used, to keep the density below that level at the busiest traffic times.
And the solution, to the rational actor showing enlightened self-interest, is to leave sufficient following distance so that rapid deceleration is unnecessary, evening out the speeds of traffic behind him or her. Don’t worry about people pulling in front of you; just gently let your following distance increase again.
Freak that traffic-nomics!
The most interesting question that comes out of this phenomenon is how to react as a driver that is at the jam point (e.g., you have been traveling at a consistent speed and have come to a sudden stop. now you are beginning to speed up again) Do you a) decrease the distance between you and the car in front of you as much as possible, decreasing delay of the cars behind you, or b) allow a cushion to develop. State of the art traffic operations technology would attempt to decrease the buffer between cars and thus maximize freeway capacity and speed. This only works when there is (fictional) automation involved, as cars interface with eachother and the road. Until this miraculous technology is perfected, I go with option “B” , so as to guage the appropriate freeway speed and proceed accordingly. That way, lane changes, sudden stops, road hazards, and everyday variety @-holes don’t force me to stop, and anyone behind me that is following my lead drives a comfortable, consistent 25 mph instead of 45 and stop like everyone else. It seems that if everyone drove the same speed, then those types of jams would not occur. The key is scaling up the speed together.