I must say I am not surprised that it works but that it works at such a cheap rate. Looking at parking costs that seems to be less than two hours of parking. My instinct would have been that there would be no effect below say ~30$.
People are not good at making rational decisions when hit with nominal fees.
That's exactly the reason why pricing should drive services. The underlying reasoning is so complex, that most of the time you'd never actually be able to understand why things are going for the rates they are.
1. Even the reduced $9 toll would have a larger-than-anticipated impact
2. Increasing the toll to $15 would have a smaller-than-anticipated impact
i.e. the presence or absence of a toll matters more than the dollar amount (obviously to a limit)
We'll see about #2 when it increases to $12 in 2028 but #1 seems spot on!
There's other factors in demand for trips, like congestion went way down during the peak of covid, and typically drops during a recession, and return to the office increases congestion, etc. But mostly it's more people; hold population steady and congestion will be more stable.