Ever since I first heard the Treasury Secretary Ken Henry raise the possibility of introducing a road user tax I haven't been able to stop thinking about how it would work.

Governments around the world have the universal problem of trying to fund public transport infrastructure and as cars become more fuel efficient, it could impact on fuel excise revenue.

The argument being presented at the moment is that we already pay for how much electricity, gas, phone and water we use so why not pay for roads as well ?

One suggestion is to remove traditional fuel and registration taxes and replace them with something more like a 'mileage tax.' The Association of Consulting Engineers Australia recently suggested that users could be billed monthly transport accounts.

Similar ideas are already being considered overseas.

The US state of Oregon has trialed the concept, fitting hundreds of cars with GPS monitoring devices. The 'black box' system recorded how many kilometres drivers clocked up, then charged them accordingly when they filled up for petrol, even more if they drove in peak hour.

The idea has also been trialed in Seattle; while drivers in London pay a congestion tax which is policed by cameras.

But paying as you drive has not been popular everywhere - the issue of a congestion tax was so contentious it was put to referendum in Manchester last year.

It conked out as drivers rejected the idea.

And of course there are privacy issues. Imagine someone tracking where you go every time you jump in your car...

It will no doubt spark questions: what if information about where you've been, what time you were there, how often you drove there, fell into the wrong hands? What if that information was used to track people down?

If you've got nothing to hide, you might not be worried and of course there is a plus side - police maybe able to use the information to catch criminals. They already do it now with RTA cameras.

Here in Australia, GPS technology is already being used in the trucking industry because it says it wants to improve productivity and keep drivers safe.

Some might say fair enough if you are running a business, but should people in the privacy of their own cars have someone knowing where they're going and what time they arrive and leave?

There are also social equity issues such as: should pensioners have to pay to use roads every time they hop in their car?

In some overseas trials, people have been deterred from driving during peak hour traffic which therefore eases congestion.

That is fine if you have a good public transport system as an alternative. But as one person pointed out to me this week, there's not much hope of that if you live in Sydney, particularly in the north west.

Jumping in your car has always been about freedom, remember the feeling of driving on your own for the first time when you got your P plates?

If this idea gets traction, maybe your big brother will come along for the ride.