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## Why Traffic Jams Can Occur For No Apparent Reason

by | Apr 25, 2018

Traffic Jams are the Worst!

The other morning I was driving with my daughter to drop her off at her school about 15 miles away from our house. As we were traveling westbound on I-64 at about 50-60 mph, all of the sudden slowed down to a crawl. After stop and go for a few minutes, speed picked back up to over 50 mph. There was no apparent reason for the slowdown –no wreck, no construction, the sun didn’t seem to be a factor and there was no major entrance or exit ramps.  My experience was not unique – it is a common phenomenon that we all experience regularly.

Traffic jams that occur for no obvious reason are called “phantom traffic jams.” A team of researchers from MIT, Temple, McGill and U of Alberta have been working on modeling why these phantom traffic jams occur. Their modeling shows that above a certain level  of vehicle density on a road small perturbations in a uniform traffic flow build up and cause a wave of higher density of vehicles. “The instabilities are observed to grow into traveling waves, which are local peaks of high traffic density, although the average traffic density is still moderate (the highway is not fully congested). Vehicles are forced to brake when they run into such waves.” These waves are similar to the shock waves caused by an explosion and the researchers have named traveling traffic waves “jamitrons.” Jamitrons are traveling traffic waves and are a cause of phantom traffic jams. Link to their research: Traffic Modeling – Phantom Traffic Jams and Traveling Jamitons

Here’s what the researchers have found about jamitrons (quoted from their paper):

• Sharp traffic shocks always face towards incoming vehicles.
• Jamitons always travel slower than the individual vehicles so that vehicles run into a sharp and sudden increase in density which forces each vehicle to brake very suddenly.
• Then, vehicles accelerate again out of the jamiton.
• Jamitons are stable structures. They can only vanish by strong smoothing effects (extremely cautious drivers) or a lowering of density (a widening road, vehicles exiting).

Here is one of the simulations of a jamitron from the researchers:

Also from the research paper, here is an experiment performed by Japanese researchers showing how a jamitron can occur even when drivers are all trying to go the same speed:

The conclusion of the researchers is that “phantom traffic jams are not necessarily caused by individual drivers behaving in a ‘wrong’ way. In fact, they can even occur if all drivers behave by the exact same laws. In the considered traffic models, two key effects work towards the occurrence of phantom traffic jams: first, denser traffic travels slower; and second, it takes a certain ‘adjustment time’ for drivers to react to new traffic conditions. These effects are counter-acted by a certain tendency of the drivers to drive preventively. In light traffic, the good effects dominate. In heavy traffic, the bad effects prevail. Hence, phantom traffic jams are a feature of traffic flow that is not completely avoidable.

While there are some things that can be done to reduce phantom traffic jams, such as better road design, greater road capacity and introduction of variable speed limits, likely autonomous cars provide the best hope to prevent phantom traffic jams.

Here’s a related IFOD on the traffic phenomenon called Braess Paradox which concerns how closing a road can actually decrease traffic congestion: Braess Paradox

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