Building an extra road sounds like an unambiguous improvement. Braess's paradox shows why that intuition can fail when drivers independently choose the route that seems fastest to them. A new link changes everyone's incentives, and the resulting route changes can overload roads that previously worked well.
The first act shows an equilibrium on two available routes, including each route's travel time and the system-wide total. The second adds a connecting link and lets drivers choose again. All three available paths can carry traffic, yet the new equilibrium has a larger total travel time. No driver acting alone can improve by switching, but the group is worse off than before the construction. The video does not claim every new road is harmful. It isolates the mechanism: network capacity and network performance are not interchangeable when route choice feeds back into congestion.
Many commuters want to arrive at a similar time, but a bottleneck can admit only a limited number of vehicles per minute. If everyone departs near the preferred time, a long queue forms. If departures are spread out, the queue shrinks, but some people arrive early or late instead.
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Drivers consider the time they personally spend on a road, but each additional driver may slow down many others. That difference between private and collective cost can make the route pattern chosen by individuals less efficient than the pattern that minimizes total travel time.
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Route choice is learned through experience. If one road becomes slower than another, some travelers switch the next day, changing congestion on both roads. Repeating this adjustment can eventually bring the travel times of used routes close together.
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