I have two problems with the recent Lib Dem policy announcement about using road pricing to lower fuel duties and fund spending on infrastructure for more "environmentally friendly" forms of transport. The one, which I will return to in another post, is about the difficulty of solving two problems - paying for roads and trying to force people off them - with this one policy. But for now I want to suggest a solution to those many commenters on the Lib Dem Voice thread that any implementation of road pricing is going to be necessarily an intrusion on our privacy.

In fact, the technology has been around for five decades: the flight data recorder, or "black box". It even ought to cost less as it would mean no additional physical infrastructure such as ANPR gantries or roadside transceivers.

Take a regular GPS Sat-Nav system. Already the technology is being developed to deliver all sorts of content to such devices (see the "Sat-nav for people" section on this BBC Click report). It would be a small step to link this to a billing system in the vehicle that got data about the current price of the road you are travelling on, and on other alternatives to help you make up your mind about what route to use, and to calculate a total bill for a journey and initiate a payment transaction without even telling the billing authority where it has been.

Ah but, people say that's open to abuse or tampering to avoid bills on the one hand, and because there's no central information about how your bill is made up it would not be possible to dispute a bill on the other. Well, this is where the "flight data recorder" comes in. You do have the details of your journeys stored, but not centrally, rather in a box in the vehicle. A box say that has to be audited as part of your annual MOT perhaps. And that can only be accessed when security information is provided by both the person or authority wanting to read it and the owner. That way, if you think it is to your advantage to disclose where you have been, for example to dispute a bill, you are in control of when that data is disclosed.

Again, this technology is already around, and in applications much smaller than aircraft. My security guard in the hall of residence has a little device called a "Deister" which they use to "prove" that they have been doing patrols. There's no live link snooping on where they are going, but the Deister gun will be audited and has logged a patrol if there is any dispute.

Can anyone see any other objections to such a way of doing it non-intrusively?