If the McDonalds employee has to pay $50 in fees to get to their job, it changes their financial calculations around whether that is a viable job. If enough of them decide to quit because of this, the restaurant will need to pay its employees more, subsidize their commutes or shut down.
If the restaurant shuts down, then it didn't make sense for them to be in an expensive urban hub, and the negative externality of the traffic their employees generated is gone. I suspect a lot of only-barely-viable jobs would go this way, and that's where traffic reduction would come from.
If they pay more, or subsidize commutes, they are paying to paying for the negative externality of filling up the highways to bring their employees in. The people who want cheap cheeseburgers in an expensive urban hub will decide if they want to pay more for cheap cheeseburgers to cover the cost. If they don't, it doesn't make sense for it to be there and the traffic isn't worth it.
There are, however, a lot of people for whom that $50 more in weekly commute costs, would be financially deadly and almost unavoidable.
It's easy for me to advocate for congestion charges; they clearly benefit me. Those who make a lot less than I do might feel very differently.