I live in a rural area, between a major population center and a major resort area, with one major highway and a few small back roads that provide alternate paths for part of the highways route. Every summer weekend the highway becomes highly congested.
Google quickly starts routing people down the back roads because of a 30 minute delay on the highway. A sudden crush of cars hits these back roads, and they end up gridlocked for 3-4 hours. Google then realizes traffic is literally stopped on these roads, and stops sending new traffic down those routes. But the people already on them are still stuck for hours.
It gets smelly when a bunch of drivers take a shit on the side of the road because they can’t go anywhere else, and leave it there.
All because Google simultaneously made an ‘individually optimal’ decision for a whole bunch of individual drivers at once.
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Another example in the same area actually causes a backup on the highway itself. Google started suggesting one back road that required an unprotected left turn across oncoming traffic on the highway, to avoid a 10-15 minute delay further on the highway. Drivers dutifully followed directions by getting into the left turn lane.
The drain rate of the left turn rate is slow because oncoming traffic is also high. The left turn lane fills up, and one driver with directions to turn then stops in the traffic lanes to wait for room to get into the turn lane. And suddenly the highway is now encountering 2-3 hour delays that don’t clear for most of the day.