Here's a thought.  Since bitcoin enables semi-anonymous transactions, micropayments for road usage are in fact possible without tracing the car or driver and without any government database.

All drivers need bitcoin.

A driver would set out on a section of road, placing a square bar code on his dash.  The system camera reads it and has authority to charge small amounts.  Then as he passes checkpoints the system docks the public key by incremental amounts, say 0.50 USD in bitcoin.  At the end of his drive, he's been billed say 3.50 in a half dozen transactions.

The way to sell this to the government agency would be to emphasize that their profit would be higher and their costs lower.

For discussion purposes let's say no license plate scanning is done except if the square bar code is not there or is not readable or does not have money.  In these cases the plate is scanned so a bill can be sent in the mail.