The great Wikileaks scandal is almost all about opening up the hidden world of international diplomacy. But, to my surprise, the leaked embassy cables do cast light on one of the most controversial aspects of British transport policy: congestion charging.

It's long been known that the US Embassy has refused to pay the congestion charge. In 2006, then-London Mayor Ken Livingstone (never America's biggest fan) finally lost his temper and described the then-US ambassador Robert Tuttle a 'chiselling little crook'.

Livingstone added that "We will find a way of getting them into court either here or in America. We are not going to have them evade their responsibilities."

Last August a new ambassador sent by President Obama also refused to order US embassy staff to start paying the charge and also refused to settle the £3.4m bill for back-payments and fines.

One of the leaked cables (sent from London almost three years ago to the day) explains why it wouldn't pay the Congestion Charge.

"The city considers this a fee for service (improved transportation infrastructure, decreased pollution and congestion), and did not grant a diplomatic exemption.

After determining that the fee was actually a tax, and therefore not payable under the Vienna Conventions of Diplomatic and Consular Affairs, the Department of State engaged in lengthy negotiations with the Foreign and Commonwealth Office and the City of London.

In July 2005, after negotiations concluded unsuccessfully, the Department instructed the Mission and its members to stop paying the tax."

Interestingly, the cable goes on to explain to Washington that '23 of the 27 European Missions' were also refusing to pay 'the tax'.

So why, as the charge approaches its eighth birthday, hasn't the US and other non-paying embassies been taken to court?

The reason is that a court judgment supporting the US stance could prevent road tolls from ever gaining wider traction in the UK.

If the courts decided that the C-Charge was indeed a tax and not a charge for a service, future road tolls would not be classed as a business expense. And that means the cost couldn't be offset against tax.

Clearly, what little support there is among users for the London C-Charge would be swept away if businesses and drivers had to pay the £8 daily fee from their wallets.

It seems then that the US embassy's 'bill' will continue to climb by millions of pounds, but it will never be paid. Transport for London will never risk a court case that could collapse the pretence that by paying £8 per day, drivers are getting a 'service'.

A service that, by Transport for London's own figures, can neither guarantee better journey times or even less congestion.