Traffic flow, ‘jamitons’ and trucks

…I like the word jamiton*… this idea of a traffic jam “particle”… a component of a deceleration wave travelling backwards through traffic from something concrete (like an unseen accident up the road) or some less tangible thing (a coincidental set of interactions between different drivers and conditions of the road)… all the way down the row of cars. There’s quite a few writeups about the phenomenon.

Researchers have discovered that pileups (or rather, holdups) can spring from nothingness; a rapid magnification of a glitch in an otherwise smooth and fairly homogenous flow of cars-as-particles. But since modelling traffic as a fluid is so common, it’s surprising to see how little is said about the granularity of that fluid.

A hypothesis:

I know nothing about fluid dynamics, but I do love trucks (by which I actually mean tractor trailers), and I’ve been observing how they move in traffic, which is how I got thinking about this whole topic.  Also, tractor trailers have big noses and look adorable. But they aren’t just cute, damnit. I’ll tell you why.

Such elephantinely-proportioned vehicles carry a radically different momentum, as you might expect, but the only component of that momentum of interest here is velocity. Thanks to their risk-averse owners and drivers, trailers don’t go super fast, even on a highway. Couple this relative, intentioned sluggishness with the horizontal space that trailers take up, and then couple that result again with the average truck-driver’s less-reactive driving profile (less slamming on the brakes, less revving up), and you have the ultimate traffic-jam-diffusing missile.

The mechanics of it is quite simple. Large trucks, lorries and trailers create space. Not just space space, but I mean space in the temporal(-ish) dimension, as well. They each allow a number of average vehicles to plan, plot, steal, hide alongside and otherwise strategise their way out of sticky situations and into fortuitous gaps (which gaps have usually also been created around these same trailers). ┬áSo, larger, slower vehicles introduce … something… I don’t know what… into the fluid flow, creating pockets into which local traffic can flow so that the deceleration wave is attenuated somewhat. (It is, after all, a sort of compression wave… so the less cars about when the wave gets to a particular point, the less well it can be propagated beyond that point).

An extrapolation:

No, I’m not going to say fill the roads with diesel-guzzling trucks to save us from traffic woes. I’m not quite that silly. But if someone could properly test my truck/trailer hypothesis and it turned out to have some truth to it, maybe the real problem on our roads (other than culprit #1 – rush-hour effects and the fact that the entire economy seems, a tad foolishly, to require everyone to turn up at work at 9 and shove off at 5)… the real problem on our roads just might be that all our vehicles are too similar in profile. Too many of them can be modelled by the same type of particle in a “fluid flow” analysis of our roads. Too many lookalike sedans (barring a few exceptions, car design in the last few decades has just been a yawn), all moving the same way, all driven by reactionary, sensory-hobbled humans, all taking up the same amount of space. If trucks/trailers are useful pattern-interrupts on the road, then by extension so are motorbikes and probably even smartcars. Maybe we need more of this kind of variation…

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*jamiton, yes – I make up a lot of words but not this one. It is actually used by some people studying traffic flow…

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