Expanding highways increases throughput almost everytime. Certainly, it rarely ends congestion. It often reduces congestion, but that depends on how far the capacity is behind the actual demand.
In most places I've lived, road expansion is a response to increased demand from increased population, either through density or sprawl (but usually both). If you let population increase without increasing tranport capacity, you get congestion. That's true for roads and mass transit. Ideally, as the population grows, you expand both. But, expanding roads is often simpler, as you can do it in parts, addressing bottlenecks one at a time, whereas mass transit isn't very effective with piece-wise solutions; I can't easily drive to a highway transit station, take that 10 miles, then drive to my final destination, I need at least one of the local trips to be on transit/walkable and have parking available or have favorable conditions for biking (weather, physical ability, room on transit, managable hills).