The Road That Slows Everyone

The board draws four roads, a shortcut, and the two headline numbers: 65 minutes before the shortcut opens, 80 after. This lesson works through exactly why, with a small example and exact numbers, and then computes the one thing the board skips: how much better the same network could have done if someone had routed it on purpose.

Before you start. You need to be able to add, divide, and compare two numbers - nothing past what you’d use splitting a dinner bill four ways. Every word this lesson needs, it builds in front of you.

The idea in 30 seconds

A network can get strictly more roads and still make every driver slower, with nobody behaving badly or making a mistake.
Every driver settling on their own best route, given everyone else’s, is an equilibrium - and an equilibrium isn’t automatically the network’s best possible outcome.
Taking the shortcut beats every alternative no matter what the others do - a dominant strategy, which is why no group promise to avoid it can hold.
One driver’s choice making someone else’s trip a little worse, without paying for it, is an externality: the actual mechanism behind the paradox.
The best the network could do with the shortcut, routed on purpose, is the social optimum, and it beats not having the shortcut at all. The road was never the problem; who ends up using it is.
Closing a road can speed traffic up too, but the famous cases are tangled up with people simply making fewer trips, a different effect entirely.

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