Because it is an absurd idea may well be your answer to that question even before you’ve reached the end of it. But bear with me a moment.

Imagine a government policy to have mandatory tracking devices in all motor vehicles, which would record all the journeys and store the data. The data would normally be private but could be accessed by the police and others if they subsequently discovered a reason to suspect someone. (You may be able to guess where I am going with this…)

It would cost a fair amount of money to implement, but the government could offer up grants to pay for the equipment, and make the installation and checking of it part of everyone’s MOT. Enforcement could follow with speed cameras upgraded to snap anyone who does not have a functioning tracker device as they go past. Sure some people will get into trying to hide their travels by somehow sidestepping or disabling the tracking, but then people doing that clearly will have something to hide – so if any behaviour like that is spotted then it can be filed in the ‘suspicious behaviour’ pile.

Oh and don’t worry – all those tracking devices will store data securely in an encrypted form and it will only be carefully authorised requests that will grant access to them.

No problem, hey? You do know just how many crimes involve cars don’t you? Most paedophiles and most terrorists have a driving licence, remember. As for car bombs, the clue is in the name.

And after all, what does an innocent person have to fear from someone else being able to build up a picture of their daily life, learning large parts of where they go to shop, whose homes they go to visit, who they frequently park near, which pubs and clubs they visit or where they go on holiday?

Well, I think you can imagine the reaction a Home Secretary would get if they announced those plans to track everyone’s car movements, and not just from those who usually worry about civil liberties…

So why is the reaction different when the Home Secretary says she wants to track everyone’s online movements?

After all, although the parallel between my thought experiment and the Draft Communications Data Bill is not perfect, if anything the unthinkable (tracking all cars) is less intrusive and less prone to data being wrongly used than the internet proposals.

The internet proposals involve the precise details of which pages on which websites everyone has visited and who they have communicated with online. My vehicle tracking example does not go nearly as far: no record of who you travel with, no tracking of where you go to once you park your car – and with data stored separately in everyone’s individual car, there is much less scope for abuse of it than ISP databases which can be accessed via one dodgy authorisation.

What is more, the vehicle tracking idea has if anything a greater scope for tackling crime than the online tracking idea. Think of all the speeding offences you could end up catching, not to mention tracking down dodgy cars without MOTs, people who have not got insurance and the like.

Unlike my satirical rewrite of the Home Secretary Theresa May’s piece calling for online tracking to make it a call for tracking every pedestrian, the vehicle tracking one is (vaguely) feasible.

So why is vehicle tracking unthinkable and internet tracking proposed? The answer I think is a simple one.

The idea of tracking your vehicle is a much more immediate, tangible and easy to understand issue. Having ISPs (companies named with an acronym no less) collect a bit of extra data is more technical, seems less immediate and harder to fully understand.

That is the challenge for civil liberties campaigners over the Draft Communications Data Bill.