Road pricing and slippery slopes

So I heard on the radio this morning that the British government is going to start trials of road pricing, where car drivers will be taxed for every mile they drive, at a rate that varies with how busy the road is. So a busy street in central London during rush hour could be taxed at up to £1.34/mile, while a deserted rural road would be taxed at only 2p/mile. The idea is to give people incentives to avoid using the most congested roads, to avoid total gridlock in years to come. (This is in contrast to gas taxes, which gives people incentives to drive less and use more energy-efficient cars, but still allows them to drive in congestion-producing patterns.)

It seems like a good idea, at least in theory, though I think it shouldn't replace the gas tax. But I'm very concerned about the so-called Big Brother objection - the government will know where your car is at all times, because without that knowledge, there's no way to tax you accurately on road usage. (You might have to install a GPS bos on your dashboard, to be read by satellites, or video cameras could snap pictures of your license plate as you drive by.) That's really quite creepy. And not only could a government abuse such knowledge, but criminals could hack into the road usage databases and track down people's cars (e.g., if a mobster wanted to assassinate someone). Now, I'm sure we could design ways to prevent abuse of the database, but once the enforcement infrastructure is set up, it will become much easier for the government to use the information for civil-liberties-destroying purposes. This is what Eugene Volokh calls a cost-lowering slippery slope.

All in all, I think road pricing is too intrusive. Better to use more traditional methods like tollbooths and London's congestion charge.