Eli5: it’s said that creating larger highways doesn’t increase traffic flow because people who weren’t using it before will start. But isn’t that still a net gain?

Question

If people are being diverted from side streets to the highway because the highway is now wider, then that means side streets are cleared up. Not to mention the people who were taking side streets can now enjoy a quicker commute on the highway

In: Engineering

Answer 1

Probably not what op had in mind, but the first thing I thought of was braess’s paradox: in specific scenarios adding (or expanding the capacity of) a shortcut can lead to longer total transit times. Conversely, removing network capacity can actually speed transit up.

It is a phenomenon of game theory.

Imagine the following routes from point a to point d:

A-b-d. Where a to b is big and fast, but b to d is slow.
A-c-d. Where a to c is slow but c to d is fast.

If you add a shortcut between b and c, opening up the fast route a-b-c-d people will (depending on the specifics) flock to that fast route, and now the whole network is congested, *including the original two paths* (because the new path includes parts of both original paths, so if the new path is congested, the whole network is congested).