Exactly. If you increase the capacity of a bus in a highly crowded area, the bus will still have 100% occupancy, but that doesn't mean it didn't help. The throughput increased.
In other words, with added lane on highways, it’s sociological mechanism, which is slightly less precise analogy.
This implies that on the first day a lot of people either didn't make their trip because there was no space, or took a different mode of transport. Presumably mostly the latter. With the highway example the same is true, just that it will be mostly people not making the trip at all before the capacity improvement because they were unwilling to make the higher time commitment