An excellent question. The "traffic engineer" profession is deeply broken. https://www.nytimes.com/2023/01/06/us/widen-highways-traffic.html
Widening Highways Doesn’t Fix Traffic. So Why Do We Keep Doing It?

With billions of dollars available to improve transportation infrastructure, states have a chance to try new strategies for addressing congestion. But some habits are hard to break.

The New York Times

We now have overwhelming evidence from a million different directions that sprawly car culture *doesn't work*. Suburbs don't pay for themselves. More lanes don't decrease traffic. Stroads kill people & inhibit economic growth. Parking is economic deadweight.

We refuse to learn.

The whole car/sprawl complex has reached such size & momentum that it just thunders on despite all this evidence -- one of many ways the US punches itself in the face over & over again while bragging about exceptionalism.
The one real change that dopey car-addled politicians & traffic engineers *have* embraced is ... replicating the problem underground, in tunnels.
@drvolts it's common sense, but whenever you're digging tunnels (high cost) instead of using the surface, whatever you're building better be worthy of that cost; more roads for cars simply do not cut it
@drvolts I think that the problem is not that we refuse to learn or that politicians are addled, but that small perturbations all have negative impacts on large numbers of people. When you eliminate parking or lanes of traffic, the average person ends up spending a lot more time doing things that they would rather not, and the public transit is not there to take up the slack.
Only a big program, comparable to the interstate highway system, is going to seem like progress.