How would limiting access by adding a cost not obviously impact people differently based on their income? I'm struggling to see why this would even need to be verified by research.
What's not obvious is how strong the impact is, does it cause substitution of travel modes or just decrease in travel, how does it change when travel takes place, what share of people are those who still have to drive etc.
In general cost, travel time, difficulty of parking, etc. all affect optional travel. (And, also, just age and inclination.) I know I'm way less inclined to go into Boston/Cambridge for something in the evening for some combination of those reasons than I used to be. I can take commuter rail in for a day event but it's pretty much a non-starter for something in the evening.
