There's a discount for low income residents from what I recall (although I believe it's small). I agree that SUVs and pickup trucks should be hit harder, but perhaps that can be added down the line.
Low-income drivers will get 50 percent off tolls during the day after the first 10 trips in a calendar month. It will also be much cheaper to drive at night: Between 9 p.m. and 5 a.m., fees will be reduced by 75 percent.
And:
people whose primary residence is inside the tolling district and whose income is below $60,000 would be eligible for a state tax credit equal to the amount of their tolls.
On the car model point, perhaps SUVs and pickup trucks could be pushed into the "small truck" category, raising the fee to $24. These "cars" exist due to dodging the efficiency standards that apply to normal cars, so they should be treated as the light trucks that they actually are.