Posted by: zyxo | November 23, 2009

Thoughts on Traffic Jams

Traffic Jam in Delhi
Image via Wikipedia

I am sure everybody knows the feeling when you get stuck in a traffic jam. No need to say this is becoming a huge problem.
Why are there traffic jams ? Is it possible to prevent them ?

What is a traffic jam ?
Very simply put : you experience a traffic jam, when there is no space in front of you to move on. We all love an empty road ahead. But you do not really need an empty road in front of you. When the driver in front of you nicely drives on, he is constantly making the necessary space so that you can move on too.
So there are two factors : i) there is a car in front of you and ii) it is not moving.
(“Ants have no traffic jams !” Are they more intelligent ?)

How much space do you need ?
This is not so simple. It depends on your speed. You want enough space to have the time to stop when the one in front of you stops. Hence you only move on when there is more space before you than the minimum you feel save with. What you really want is not space, but time. A good -conservative- rule of thumb is 4 seconds or 2 crocodiles (just say : “one crocodile, two crocodiles”).

To put it the opposite way : When is there no traffic jam ?
First everybody must be moving, and second there has to be enough time between the cars.

How to prevent traffic jams?
Since there are two factors in play : space and speed (space/time) we can play with both.

i) The first is space : it is obvious that lowering the number of cars on a given time on a given road will be a good thing, making more room per car. So you need to prevent (some) people to take their car, for example by enhancing public transportation, by making it more expensive to drive a car (taxes).

ii) The second one is less intuitive : a general remedy to traffic jams is limiting the speed. Why ?
My first reaction is : this makes no sense at all! If at a high speed or a low speed you allways keep 4 seconds between two cars, this means that either way every 4 seconds there is a car. So at a lower speed the road cannot “transport” more cars per time-unit.
However, there is another consequence of driving slower : the space you need in front of you diminishes : 4 seconds at 70 km/hour means that you need 77.8 meters, but at 120 km/hour you need 133.3 meters. So the effect of speed limitation is that the road can contain a lot more cars : 12.8 per kilometer at 70 km/hour, compared to only 7.5 per kilometer at 120 km/hour.
So either lowering the number of cars or limiting the speed leads to the same consequence : it prevents saturation of the roads. However, from the moment on that the road is saturated, the same traffic jam misery will start again.

iii) A third solution would be to lower the distances between the cars without changing the speed. Sure, there would be a security problem, unless everybody becomes an extreme alert driver (like the formula 1 people). A (future ?) solution is electronics. We can easily imagine a device with sensors to keep automatically a minimum distance. In stead of our automatic cruise control, we could switch to automatic distance control : This already exists ! I remember there has been an experiment like this with trucks, with only one driver in the first one and the other trucks simply automatically follow everything the first did, just a few meters separated from one another. Here is a more recent article on a similar subject.

And what about the “mystery of traffic jams” or “phantom traffic jams” ?
This is not really a mystery or a phantom, it’s just the result of a saturation of the road and the behaviour of the drivers.

Anyway : the best way not to get stuck in traffic jams is to stay at home !

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  1. You know it had never occurred to me that slower moving traffic can support a higher density of cars, but when you explain it, it makes complete sense!

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