Opening space for traffic lanes does not reduce congestion, it induces demand (for more traffic lanes).
I'm not even sure it is possible to satisfy this demand in a city. Houston has 26-lane highways and they are still building more roads to handle traffic. LA uses 14% of all land for car parking and is still building more parking spots. I suppose there must be a theoretical upper bound past which building more space for cars does not eventually induce more driving, but I know of no city that has reached that limit.