Suppose you live in a small village just outside a city, and every morning you commute to work (along with 9 other residents) by one of two routes linking the village and the city, one via Town A and one via Town B.
On route A, the first portion of the road is relatively short, but narrow, and so takes N minutes to complete, where N is the number of cars taking the road. The second portion of the drive is longer, but with a much better carrying capacity, so takes a flat 12 minutes.
Route B is the opposite, with the longer, wider section first, and the shorter, narrower section second. Assuming that every driver wants to make their drive as short as possible, 5 people will take route A, and 5 will take route B, giving a journey time of N+12 = 5+12 = 17 minutes.
The council thinks this isn’t ideal, too many traffic jams on the narrow portions they say. So they add a road connecting Town A to Town B directly that takes 0 minutes (they’re really very close together). People have the option to change routes halfway through, however something very strange happens.
Now, when leaving the village, people deduce that the first portion of A is going to take 10 minutes tops (with all 10 cars on it) whereas the first portion of B will take 12! Of course they choose route A. Then they see that the second half of route B is again, at least 2 minutes quicker than the second part of A, a no-brainer. So everybody takes the first part of A, followed by the second part of B, however, the average travel time has now gone up to 20 minutes, 3 minutes longer than before.
Increasing the capacity of the road network has made the average commute slower! This is not a purely mathematical issue, and has been observed in cities around the world.