Transport Secretary...

Transport Secretary Alistair Darling is keeping a close eye on a trial project in Portland, Oregon, US - and could use it as a replacement for car tax if it proves a success.

Motorists who sign up to the pilot have a black box installed in their car, and pay monthly bills for their mileage. In return, they get a discount when fill-ing up. According to the Energy Savings Trust (EST) - a Government-funded agency - the new scheme could save billions of pounds if used instead of hi-tech alternatives.

"Elements of the Portland scheme could be adopted here, making road pricing cheaper and easier to implement and run," said an EST spokesman.

The pilot also solves fears about privacy, as the black boxes won"t spy on drivers. The devices will only know when a vehicle crosses into a different charging zone, and do not record any other details about journeys.