– What causes traffic jams?

The Japanese Mathematical Society of Traffic Flow set up this unique experiment to figure out why traffic jams appear for no apparent reason (see video). New Scientist explains:

Traffic that grinds to a halt and then restarts for no apparent reason is one of the biggest causes of frustration for drivers. Now a team of Japanese researchers has recreated the phenomenon on a test-track for the first time. The mathematical theory behind these so-called “shockwave” jams was developed more than 15 years ago using models that show jams appear from nowhere on roads carrying their maximum capacity of free-flowing traffic – typically triggered by a single driver slowing down.

After that first vehicle brakes, the driver behind must also slow, and a shockwave jam of bunching cars appears, travelling backwards through the traffic. The theory has frequently been modelled in computer simulations, and seems to fit with observations of real traffic, but has never been recreated experimentally until now.

There’s more explanation at the New Scientist web site.

Temple University maths Professor Benjamin Seibold, quoted in this CBS report, says “phantom traffic jams” are no one’s fault:

At high traffic volumes, any  small disturbance in traffic flow can trigger a ripple-effect of drivers hitting the brakes, creating what he calls a “phantom traffic jam”. There’s no outside reason for those traffic jams.  There’s no obstacle on the road.  There’s no car in the breakdown lane.  They’re an instability in the traffic flow, so they can occur without any visible reasons.

He told CBS reporter Mike DeNardo that “the wave can travel backward for miles, and spawn successive waves…… individual drivers can mitigate the effects by driving less aggressively and leaving a little more distance from the car in front — assuming another driver doesn’t cut into that space you just created”.


2 Comments on “– What causes traffic jams?”

  1. TomD says:

    Anecdotally, this was already commonly known just by observation alone by those driving on the freeways of the US. And had already been explained in these terms by other investigators seeking an explanation. The slowing down, stopping and starting processes all start a chain reaction of their own.

    Even when an accidents stops or nearly halts traffic going in one direction on a freeway it is almost certain the traffic will also be severely slowed if not halted in the adjacent sets of lanes travelling in the opposite direction in the US, because of what is called the looky loo syndrome. Basically drivers slowing down to look at the accident and then causing a continuing slowdown behind them, made worse by many of the subsequent also taking their own look!

    Ditto for all converging freeway points and lanes where merging, although required to be done at high speed, always causes some slowing when trying to fit within the available or unavailable ‘merging space’ between vehicles.

  2. Andrew says:

    From a anecdote I once heard: In Melbourne when traffic starts off from a traffic light each driver waits for the car in front to move and then he/she moves off. In Italy and France the drivers all move off from a red light as one. The lights turn green and everyone starts at the same time. I expect it is god help anyone who doesn’t and safe stopping distances don’t come into the equation. Melbourne’s traffic might be described as a piece of elastic, whereas the afore mentioned, quite rigid. I would guess they travel on motorways in the same manner, which may or may not mean less traffic waves.


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