Does building a big fast road between two towns make the traffic go faster. You would think so but it is not always the case.
Imagine that you live in a place called Greenville and you want to get to Springfield. You have two choices you can either go via Franklin, or via Clinton. There is a slow road from Greenville to Franklin and a fast road from Franklin to Springfield. The other route is the opposite way around and has a fast road to from Greenville to Clinton and a slow road from Clinton to Springfield.
This looks like this:
The two routes take the same amount of time overall, one is fast then slow, and the other is slow then fast. This means that some people choose to go via Franklin and some via Clinton but the traffic splits equally between the two routes.
Now the highway planners decide that to speed things up they will build a super-fast highway between Franklin and Clinton.
Surely adding a very fast road to a network must speed up the journey time. But now we have this situation:
What happens now is that because they can travel on the new super fast road it is in everyone’s interest to take the fast road from Greenville to Clinton, use the super fast road to get from Clinton to Franklin and then take the fast road from Franklin to Springfield.
Now, instead of splitting the traffic over two different routes and keeping down congestion everyone takes the same route. This means that this new route ends up being congested and actually slower than the original route.
Adding a new super fast road has encouraged the traffic to all follow the same route. They can’t even benefit from going back to the old route and use the slow road from Greenville to Franklin because they then find that the road from Franklin to Springfield is congested will all the other cars. Going from Greenville to Franklin, using the superfast road from Franklin to Clinton and then the slow road from Clinton to Springfield doesn’t work either because then they have to go on two slow roads.
Adding a route actually slowed down the traffic. Sometimes it is better to give people multiple slower routes than one fast route that gets congested.
This is known as Braess’s Paradox. If you are interested in the mathematics of the problem then check out this link.