Expanding highways is sold as "making traffic better", or "scaling to day-to-day traffic" as the grandparent poster put it.
However, what expanding highways actually accomplishes is incentivizing people to move out of the city because they can have a commute which is longer in miles but shorter in minutes. That lasts until enough people move to the suburb that the commute is now longer in miles AND equal time or longer in minutes. You haven't made traffic any better, you've just made sprawl worse and you've increased the vehicle miles traveled to accomplish the same result of getting people where they want to be.
The message of the New Urbanists is that we should make our cities more livable and build/allow more housing units within the cities so that more people can live where their commutes are short (in terms of miles) and the increase in density will make providing transit more cost-effective.
However, what expanding highways actually accomplishes is incentivizing people to move out of the city because they can have a commute which is longer in miles but shorter in minutes. That lasts until enough people move to the suburb that the commute is now longer in miles AND equal time or longer in minutes. You haven't made traffic any better, you've just made sprawl worse and you've increased the vehicle miles traveled to accomplish the same result of getting people where they want to be.
The message of the New Urbanists is that we should make our cities more livable and build/allow more housing units within the cities so that more people can live where their commutes are short (in terms of miles) and the increase in density will make providing transit more cost-effective.