[roel_v]

Nr of cars vs congestion follows an S-curve with a very narrow/steep middle section. Removing a single digit percentage of cars/passengers can mean the difference between 'busy but only a little slow down' and 'complete stand still'. I don't know about the numbers in this specific case, but transport systems work in not very intuitive ways.

[SideburnsOfDoom]

Indeed and adding this capacity could easily make things worse with induced demand.

https://en.wikipedia.org/wiki/Induced_demand https://www.wired.com/2014/06/wuwt-traffic-induced-demand/

[SECProto]

While induced demand is definitely a thing, they're not going to start selling more tickets to baseball games because the road to the park isnt quite as congested. Induced demand is because of the effects it has (encourages more spread out development and 1PV usage).