More on road physics

Following on from the last entry, another example of symmetry breaking on the roads is the stop-start kind of traffic jam that forms in heavy traffic on a motorway. When sitting in a traffic jam caused by ‘sheer weight of traffic’ (affectionately known as SWOT to traffic-analysts) you might, like me, have been inclinded to think something like "if everyone gently moved forward at exactly the same time we could get the whole thing moving along". And that would happen, if everyone accelerated evenly at exactly the same time. That’s a case of  ‘symmetric’ traffic flow – everyone is doing the same thing.

Of course, as we know, in practice under SWOT conditions, there is bunching. A car brakes for some reason, the car behind brakes harder, and a few hundred metres behind the traffic grinds to a halt. The traffic in front of the stationary lot then moves forwards, allowing this stationary group space to get going again. And you have stop-start-stop-start all the way along the motorway. In this case the symmetry of the situation has been broken (hence ‘symmetry-breaking), not everyone is doing the same thing at the same time, even though, in theory at least, they could be.

To prevent this stop-start hassle traffic managers can make use of variable speed limits, or lights controlling traffic entering from the on-ramps. The general idea is to alleivate the need to brake, and so keep it flowing smoothly. But this is active intervention – left to its own devices, a SWOTed road will break symmetry and cause that annoying bunching.  So don’t curse those on-ramp lights – they are actually improving your journey time.

P.S. According to Wikipedia, I was wrong to assert earlier that variable speed limits have yet to hit NZ. Apparently SH1 through  Ngauranga Gorge north of Wellington has one. What do people think of it?

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