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.