This is encouraging: the Washington State department of tranportation has decided against building a highway and railroad parallel to I-5 through Snohomish County, north of Seattle.
The biggest reason: cost. Quoting the article: “According to estimates, it could take up to 50 years to build such a second north-south corridor and would cost $42 billion to $50 billion.” (Emphasis added.)
This should be a potent reminder: building new traffic capacity is becoming absurdly expensive. As land values have risen, so has the cost of condemning right of ways. Similarly, new highways are typically built to higher standards than older ones, and are therefore more expensive to build.
So whenever you hear that a new road—particularly through an already-developed area—would be a good thing for the local economy, make sure you check their figures. And hold onto your wallet.