If you cut funding to highways for a year it's not like traffic would go down. If you resurfaced every interstate at once, it's not like people are going to be buying brand new cars with magic new disposable income. At a certain point, the global demand on the roads is a fixed number. Induced demand might kick in locally, but that involves people who already have cars waiting out traffic.

That's only because highways cover the country like a fishermans net. Capacity and connectivity are massive for roads. Locally it might back up, but average the capacity of every road in the U.S., and it can probably handle the population of earth. Imagine if that were the case for high speed trains, it would be borderline idiotic to drive.

It seems like recently transit agencies are finally working to improve options for commuters beyond the road network. Simple fixes, needing only a can of paint and a traffic cop enforcing law and order, like bike lanes and buss lanes can be done practically overnight in many urban areas.

LA metro has studies done for hundreds of transit corridors involving bus lanes, BRT, LRT, and HRT. The only barrier seems to be political will, not the engineering. I'm sure this is the case for most large cities, so if we ever get an administration that would prioritize funding transit projects on a federal level, as was done a century ago, we could see a transit explosion.