The Braess Paradox is the observation that adding one or more roads to a road network can slow down overall traffic flow through it.

This paradox supposedly applies for cars on highways but can extend to other vehicles, or other applications such as internet data or electricity within transmission networks etc...

Now the problem being that I searched examples of Braess paradox across the net, and I always find the same "example network" again and again, with :

  • A diamond shaped road network between 2 cities
  • Of which 2 branches have a constant travel time and 2 branches a variable travel time
  • Adding a connexion in the middle of the diamond slows down the trafic because of the variable travel time.

The problem I have with this example is that :

  • The diamond shaped network is not very realistic, in most place the road network was initially web-shaped before highways were added in the 1960.
  • All roads have variable travel time and will eventually saturate if too many cars goes through them. There is no such thing as a constant-travel-time road, but the Braess paradox relies on it
  • The saturation of roads is probably highly non-linear, however all examples I find it is described as "travel time is X minutes where X is how many thousands of cars takes this road", this is unrealistic

Some real-life examples where removing a highway contributed to decrease trafic are mentionned. However those might just as well have been also due to other efficient measures to reduce traffic and not solely to the highway link removal. In the end it seems every article about Braess paradox I find are just copy/pase of eachother.

It is however super-easy to do a real-life experiment for Braess paradox on a real road network, either by simulating trafic or by actually closing an real-life avenue. However I never find examples of this.

Now my question: Is there some real-life example, not the totally unrealistic "diamond network" always given as an example, about the Braess paradox ?

IMPORTANT NODE : I'm not talking about other measures to encourage or discourage trafic, such as induced demand. Braess paradox is about how, everything else being unchanged, adding a highway will slow down trafic.


1 Answer 1


Here is a simple circuit that demonstrates Braess's Paradox. If the red switch is open, 1.5 Amps will flow from the power source to the ground. If the red switch is closed, adding a conductor to the circuit, the current DROPS to 1.0 Amp.

A simple circuit exhibiting Braess's Paradox

When the red switch is closed, no current flows through the Zener diodes.

This is not a theoretical example. You can build this circuit with off-the-shelf components.

  • $\begingroup$ Why would I think analog electronics components such as resistor and zener diodes model accurately road trafic in any way ? $\endgroup$
    – Bregalad
    Apr 12, 2022 at 10:13

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