Language & WorldMental model
More lanes don't make the traffic jam smaller.
Widen a road and travel time drops briefly. That very drop attracts new trips: people switch from the train, drive at different hours, move farther out. Within a few years the jam is usually back, just with more cars. Studies of US cities found lane-kilometres and kilometres driven grow almost one for one. This is induced demand.
Useful: For any capacity expansion, ask: what new use will it attract, and do we want that?
Source: en.wikipedia.org