Wonder about the road damage aspect of this. If the relationship is a 4th power of weight, assuming a single vehicle with 2x the weight, it would need to replace 16 individual cars to break even.
"According to a 2022 study from the Environmental Protection Agency, the average weight of a car is 4,094 pounds."[1]
Going from 1 person of 200lb to 4 people totaling 800lb (in a 4,000lb car) increases the damage by less than an additional car (4,800 / 4,200) ^ 4 = 1.71
Good question. Maybe the amount of toll reduction should depend on the number of people carried and the weight of the vehicle, encouraging lightweight vehicles that can carry a lot of people. Probably should have a hard cutoff too so e.g. over a certain weight loses the reduction altogether.
