Though it might be a good idea to also encourage carpooling with reduced tolls, because presumably one bigger car carrying 5-7 people is better than 5-7 smaller but still bigger-than-city-cars carrying 1 person each.
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
[1]: https://finance.yahoo.com/news/average-car-weight-140033718....