There are quite a few alternative designs that don't require an identifier that is traceable to an individual. They don't support charging based on the amount of use within a month. The user has an anonymous account to deposit money into and money would get deducted each time past a toll collector. No one has to maintain an association between account numbers and people's names. New Orleans apparently uses this kind of a system.
Drivers get individual boxes, and pay into anonymous accounts.
If anyone can give me more details about this system, I'd appreciate it. The most sophisticated system might have active transponders in the car that themselves maintained records about how much money had been deposited. The toll collector would send a packet asking the box whether it had enough money and then to deduct the current charge. When the box indicated it was running low, it could be recharged like a Pitney-Bowes postage meter. This scheme would obviously require much more security in the box and reasonably sophisticated encryption mechanisms in order to make sure that the same packet couldn't be used more than once to pay for trips, etc.