It happens frequently on the autobahns in Germany. It occurs less often in Ireland, because our motorways are less congested, but it is not unknown on the M50, I suspect, at rush hour.
I refer to the phenomenon where you have inched your way forward for a mile or more in very heavy traffic, only to find when the jam has cleared that there was no obvious cause for its existence. One assumes an accident, a sudden patch of fog, or perhaps a lengthy stretch of road-works, but when you get there -nothing!
Scientists who study these things and construct computer models to describe them have discovered that traffic behaviour sometimes seems to imitate certain processes common in the atmosphere. Matter, as we know, can exist in any one of three states - solid, gas and liquid, depending mainly on the temperature.
In addition, however, water droplets, for example, sometimes exist in the atmosphere in a "supercooled" state, remaining liquid at temperatures well below the freezing point until something triggers a sudden freezing; the build-up of rime in "freezing fog" conditions, when supercooled fog droplets collide with a solid object, is a typical case. Motorway traffic mimicks these conditions. When there is little traffic, drivers can speed or crawl, and move from lane to lane at whim without interfering much with anybody else. This, in the trade, is known as "free flow", and is analogous to the gaseous state; the density is low and each "particle" is more or less free to wander where it wills without interacting with the others.
"Synchronised flow" appears when the density of traffic increases above a certain threshold; all three lanes in a three-lane highway move at more or less the same speed. It provides a lower average speed per vehicle than free-flow, and overtaking is not easy and tends not to happen. It resembles the liquid state - a moderately high density, but retaining good mobility.
And the third type of traffic flow, a jam, is of course the solid state - very high density and no mobility at all.
Transitions from one type of traffic flow to another tend to be abrupt at their respective thresholds, just like melting and evaporation. Moreover, above a certain traffic density, synchronised flow is inherently unstable; a driver braking suddenly, or a slight constriction on the motorway, can precipitate a sudden "freeze" that spreads rapidly upstream.
In these circumstances, the dense synchronised flow resembles a supercooled fluid, where even the slightest disturbance acts like a particle of dust dropped into the liquid, providing a nucleus around which a transition to the solid state takes place almost instantaneously.