Adding a new road to a city can make every driver's commute slower
Picture a road network where drivers each choose whichever route looks fastest to them individually. Mathematician Dietrich Braess showed in 1968 that adding a new shortcut can tempt so many selfish drivers onto it that the whole network gets slower for everyone — even though no one forced any single driver to make a worse choice.
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It happens because each driver optimises only their own route, not the system as a whole, so the new road can push traffic into a collectively worse equilibrium. It's not hypothetical: closing part of New York's 42nd Street in 1990 measurably reduced congestion nearby, and closing Seoul's Cheonggye Freeway sped up surrounding traffic instead of worsening it.
— Dietrich Braess, Über ein Paradoxon aus der Verkehrsplanung — Unternehmensforschung, 1968