Traffic congestion is exacerbated by the mutually exclusive zoning of residential and business.
The more that zoning mutual exclusivity of business and residential is reduced, the more traffic congestion could be reduced, not withstanding other factors.
The further separated individuals are from what they need and want, the further they must travel to get it, creating more traffic on the roads and public transportation.
Therefore, to reduce congestion, do not build bigger roads. That only increases congestion since the bigger roads are, the more people use them. Rather, bring where people live, where people buy things, and where people work closer together. Therefore, remove zoning regulations which keep businesses and residences like homes and apartments separate.