Typically new transit projects come with promises of alleviating traffic, which works great for politicians lobbying the approval of their constituents. There is however the small matter of evidence. With few exceptions, studies show that in the long term the more space that opens up on a road, the more drivers emerge to grab it. Once a city reaches a certain level of congestion and hits a wall in terms of road space, rail or bus systems are the only way to pump more people into these central areas.

“There is a way to relieve traffic congestion: charge drivers to enter high-traffic areas at high-traffic times. Congestion pricing has worked everywhere around the world it’s been implemented. But since not everyone is able or willing to pay a road fee, congestion pricing requires something besides the right price to succeed—namely, good public transit alternatives.”

Source: Public Transit Does Not Have to Reduce Traffic Congestion to Succeed