[dragonwriter]

> Save some miracle involving housing prices, it's hard to levy a consumption-based tax that isn't regressive based on income.

You could allow income-based discounts, either retroactively by refund (with receipts + tax return) or prospectively (if you have pre-registered accounts) or by a combination (e.g., if your income for the year qualified you for a higher discount than the advance registration info indicated, you can file for a refund.)

This is obviously potentially operationally more expensive in the general case, but for a congestion charge you are going to want to push people to personal accounts and toll tags anyway to minimize toll delays, so most of the infrastructure will already be there to support both prospective and retrospective discounts.