And, more than not being a problem, the highway filling up immediately is actually a benefit. Because more people are able and are choosing to travel, to accomplish whatever they need and want to accomplish.
Granted, the externality costs of all that car travel are high. But that's almost a separate issue and one that we really should be solving by a better accounting of costs and additional taxes. It's a shame that won't happen.
Traffic speeds generally have a floor of how long the equivalent public transport journey is. (If the public transport journey is faster, people will use that instead.) The issue now is that American land use is so out of whack it’s hard to deploy transit widely enough to make that true for the majority of trips.
Downs-Thomson Paradox is also interesting here and relates to your second paragraph. In a built out environment, transit trips and car trips will take roughly the same amount of time, door to door. Transit advocates have used that finding as a way to complain about road projects, but I think it actually is a much more interesting finding that suggests a more multi-modal approach to planning. I wonder how relevant or irrelevant it is for non-built-out environments.
Usually the issue with suburban transit investment occurring over urban ones is that urban areas are so weak they cannot fund expansions by themselves, and if you want the suburbs to pony up they’ll get their pound of flesh
The problem is generally selling capacity expansion as "reducing congestion" when it does nothing of the sort. That, and the capacity gets used so quickly that you are very quickly spending yourself into a hole trying to build ever more highway capacity. The thing about a train is that it's very hard to make it so crowded the service degrades due to congestion, at least not within the first 20-50 years.
This black hole of money needed for highway congestion then starts pulling away from other municipal and transportation funding needs.