C-charge discount for drivers with black boxes

Drivers are set for big congestion charge discounts — in return for fitting electronic "black boxes" that slow down their driving.

The technology will be on sale in three years time, according to Transport for London. A dashboard monitor linked to satellite navigation software will inform motorists of speed limits of every road within Greater London.

It will warn drivers when they exceed the limit and urge them to slow down with a flashing sign or audible signal. In return for fitting the device, which TfL says will cost a "few hundred pounds", drivers are likely to get discounts on both their insurance and the C-charge.

The scheme is aimed at slashing the number of road deaths and reducing emissions but also signals an end to engineering measures designed to slow drivers, such as road humps. TfL, which is developing the system, wants London to be the first city in the world to have Intelligent Speed Adaptation. By the end of this year it will have completed a digital database charting speed limits on every road within the M25.

The first speed warning device will be fitted to a bus by the middle of next year. It will be followed by extensive trials on up to 10 further vehicles, including black cabs and private hire vehicles. In 2008-09 there will be further trials, culminating in drivers being able to buy the system from 2010. Officials stress the system will be voluntary. It will be up to a driver to slow after warning signals.

Edmund King, executive director of the RAC Foundation, said: "A system that reminds motorists what the limit is will have big benefits in terms of road accidents and emissions."

Robert Gifford, of the Parliamentary Advisory Council for Transport Safety, said: "If we can achieve reductions in fatalities, better journey times and lower emissions, drivers have nothing to fear."

Analysts say harmonising vehicle speeds would improve traffic flow. TfL studies suggest ISA could cut road deaths by 24 per cent and serious injuries by 18 per cent by slowing drivers down. This would mean 55 fewer deaths in London each year and more than 700 fewer serious injuries.