Why does adding a brand-new road sometimes make EVERYONE'S drive slower?
After you watchWhy does adding a brand-new road sometimes make EVERYONE'S drive slower?
The short answer
Adding a new road can make everyone's drive slower because each driver selfishly takes whatever route looks fastest right now. A tempting shortcut looks like the quickest path, so all the drivers pile onto it and onto the crowded roads it feeds. Those roads get slower the more cars use them, so the new road creates a jam that makes every single driver's total trip longer than it was before the road existed. This is called Braess's paradox.
Try this next
- What if there were only 2000 drivers instead of 4000 when the shortcut is open? Drop the crowd on the bridge to half before flipping the shortcut open, then predict: does the jam still happen, or is there enough room now?
- What if the bridge got wider so its time grew more slowly with the crowd? Lower how fast the bridge clock climbs as cars pile on, then predict whether the open shortcut still beats 65 minutes.
- What if you watched a real express checkout that everyone switches to? At a store, predict first which line will end up slowest, then watch as shoppers all hop to the one that looks fastest.
Now you — bend it
- What if What if you opened the shortcut but only let half the drivers use it?Think about how crowded each bridge gets if only some cars flood the new path instead of all 4000.
- What if What if the highways were slower, say 60 minutes instead of 45?The shortcut skips the highways, so slower highways make the bridge-only path look even more tempting — predict if that makes the jam worse.
- What if What if you added a second shortcut on the other side?Another tempting path means another road the crowd can flood — guess whether two shortcuts help or pile on more cars.
Can you prove it?Opening the new shortcut makes every driver's trip longer, not shorter. — Run the bridge alone and read the 65-minute clock, then flip the shortcut open and watch the average drive climb to 80 minutes — proof the extra road slowed everyone.
Design your own test:Before you change it, predict: if the bridge barely slows down when crowded, will the open shortcut still make everyone's trip worse?
Explain it to a 6-year-old: If everyone races to the same little shortcut, it gets packed and slow, so sometimes a new road makes the trip take longer for all of us.
The whole story
How it works
Picture two routes from A to B. Each route is one wide highway that always takes 45 minutes no matter how busy it is, plus one narrow bridge whose time grows with the crowd (about cars divided by 100 minutes). With no shortcut, 4000 drivers split evenly, 2000 each way, so every trip is one bridge (2000 over 100 = 20 minutes) plus one highway (45 minutes) = 65 minutes. Now add a shortcut that links the two bridges. The path bridge to shortcut to bridge skips both highways and looks fastest, so all 4000 drivers take it. Both bridges now carry every car, so each bridge takes 4000 over 100 = 40 minutes, and every trip becomes 40 + 40 = 80 minutes. The extra road made everyone slower.
What people get wrong
People assume that adding a road, a lane, or another option can only help traffic, or at worst do nothing. In fact, when every driver grabs the locally fastest route, an extra road can overload the crowd-sensitive roads it feeds and slow everybody down. The flip side is just as surprising: closing a road can speed a whole city up, which has really happened when cities removed busy roads.
The catch
Opening the shortcut is genuinely the fastest choice for the very first driver who tries it, which is exactly why it is so tempting and so hard to give up. But once everyone copies that smart-for-me choice, the bridges it feeds jam and every trip rises to 80 minutes. Closing the shortcut drops every trip to 65 minutes, but giving up a road feels like a loss, so it only works if all the drivers let go of it together.
Questions kids ask
If the shortcut is faster for one driver, why is it slower for everyone?
It really is fastest for the first driver who sneaks onto it. The trouble is that it looks fastest to everyone, so all 4000 drivers take it at once. That floods both narrow bridges, and bridges get slower the more cars use them, so the trip that looked quick ends up the slowest for the whole crowd.
How can closing a road make traffic faster?
Closing the tempting shortcut removes the option that everyone was flooding. Without it, drivers spread sensibly across the two routes instead of piling onto the same crowded bridges, so each route stays uncrowded and every trip gets quicker. Several real cities have sped up traffic this way by removing a busy road.
Has this really happened in a real city?
Yes. When Seoul tore out a six-lane highway to uncover the Cheonggyecheon stream, traffic actually improved. New York closed part of 42nd Street on a busy day and traffic got better, not worse. Stuttgart saw a similar effect. These match what Braess's paradox predicts.
Why doesn't the wide highway slow down when the bridge does?
The wide highway has so many lanes that extra cars still have room, so its time stays about the same no matter how busy it is. The narrow bridge has only one skinny lane, so every extra car has to wait its turn and the time climbs. That difference is the whole reason the paradox works.
Talk about it
- Guess first: if the city built a brand-new road, would your drive get faster, slower, or stay the same — and why?
- Why might everyone picking the fastest road for themselves make the whole crowd slower?
- Can you think of a time a shortcut everyone took ended up being the slowest way?
For grown-ups
This is Braess's paradox (Dietrich Braess, 1968). Model two paths from source to sink, each with one congestion-free edge of cost 45 and one congestion-sensitive edge of cost equal to cars divided by 100. With 4000 drivers and no cross-link, the user equilibrium splits 2000/2000 and every trip costs 65. Add a zero-cost middle link and each driver's best response becomes the all-bridge path, so all 4000 use both narrow edges at cost 40 each, total 80. The new equilibrium is worse for everyone, a Nash equilibrium that is not Pareto-optimal, the price of anarchy. Real road removals such as Seoul's Cheonggyecheon, New York's 42nd Street and a closure in Stuttgart eased traffic just as the paradox predicts.
Keep going
What else makes you wonder?
- If a road can make everyone slower, what other helpful-looking things might quietly make a crowd worse off?
- How would all 4000 drivers ever agree to skip the shortcut together when each one is faster taking it alone?
- Are there places besides roads — like water pipes, the internet, or a school lunch line — where adding a path could clog everything up?