That assumes that people will change from a car to other options. At least where I live, that will not happen. They tried to do exactly what you said - instead of increasing size of roads, they broadened roads but made the new lanes only for public transport. The expected happened - everyone who used cars before, were using them after, and only thing that improved is people who ride bus reach destination a little faster. Same would be with bikes and other things - nothing will have impact, car throughput is basically only thing that matters in traffic congestion reduction.