1Two kinds of road
Some roads slow down when they're crowded — some never do
You only need two ideas. Watch each road fill up with cars:
The wide highway
Always 45 minutes — no matter how many cars. It has so many lanes that a crowd doesn't slow it at all. Roomy and steady.
The narrow bridge
The more cars, the slower it gets. One skinny lane. Pile cars on and the time climbs and climbs. An empty bridge is quick; a packed one crawls.
And one more thing: every driver is smart but selfish — each one just takes whatever route looks fastest for them right now. Nobody's being mean. The puzzle is what happens when 4000 drivers all do that at once.
2The town, before and after
Two long ways… and a tempting shortcut
Getting from A to B, there were always two routes. Then the city added one little road in the middle:
The two long ways
Top: cross the bridge, then cruise the highway. Bottom: highway first, then bridge. Each way is one crowded bridge + one roomy highway.
The cut-the-corner road
A new shortcut links the two bridges. Now you can hop bridge → shortcut → bridge and skip BOTH highways. It looks like a genius move.
3Your turn — feel the crowding
Pour cars onto one bridge and watch its clock climb
Slide cars onto the narrow bridge and watch its time grow. Right beside it, the wide highway carries the same crowd without budging. This is the whole engine of the puzzle.
Narrow bridge20 min
Wide highway45 min
Bridge time is just cars ÷ 100 minutes. The highway ignores the crowd — always 45.
4Now build the new road
Flip the shortcut on — and let 4000 selfish drivers loose 🚗
Here's the whole city. 4000 drivers, each grabbing the fastest-looking route. You hold one switch: the new shortcut, OPEN or CLOSED. Guess what it does to the average drive — then flip it and watch.
Guess before you flip the switch
Right now the average drive is 65 minutes. The city opens the shiny new shortcut and every driver grabs the fastest-looking route. How much does the average drive change?