Thousands of taxis, buses and council vehicles could be fitted with devices that prevent them from exceeding the speed limit.

The technology — known as Intelligent Speed Adaptation (ISA) — is being tested by Transport for London in a trial starting this summer on all roads inside the M25.

Drivers of vehicles with ISA will be able to select an option that prevents them from accelerating over the limit. The vehicle will also automatically slow down if the driver fails to reduce his speed when he passes a sign marking a lower limit.

The device uses satellite tracking and a digital road map to detect the local speed limit.

In addition to the automatic mode, the device has an advisory setting in which the driver is simply informed of the limit and told whether he is complying. A “smiley face” appears on the unit if the driver is obeying the limit. It frowns if he goes too fast.

The technology is likely to be offered for sale next year to private motorists. Those with six or nine penalty points might choose to have it installed to avoid an automatic six-month driving ban for receiving 12 points within three years.

Transport for London (TfL), which will today announce a six-month trial of ISA, estimates that, if two thirds of London drivers used the devices, the number of road casualties in the capital could be reduced by 10 per cent.

The improvement in road safety could reduce congestion, a quarter of which is caused by collisions. Vehicles obeying the limit would also consume less fuel and be less polluting.

Even if only a fraction of vehicles were equipped with ISA, they would have a widespread effect on traffic speed because other drivers would have to slow down behind them. However, some motoring groups have voiced concern that ISA-equipped vehicles would result in a rise in dangerous overtaking.

TfL’s trial will include a London bus, a licensed taxi and 20 cars driven by road engineers, traffic managers and highway inspectors. If the trial demonstrates clear safety and environmental benefits, bus companies and taxi operators could either be obliged to equip their vehicles with ISA or be given incentives to do so.

Results of the trial will be published in the spring, when the technology will be made available to other organisations.

Southwark Council has expressed an interest in fitting ISA to more than 300 of its vehicles.

Chris Lines, head of TfL’s road safety unit, said: “This innovative technology could help any driver in London avoid the unnecessary penalties of creeping over the speed limit and at the same time will save lives.

“We know the technology works and now we want to know how drivers in all types of vehicles respond to it. ISA is intended as a road safety device, but if Londoners embrace this technology we may well see additional benefits, including reduced congestion as a result of collisions and reduced vehicle emissions as drivers adopt a smoother driving style.”

Jeff Hook, executive member for the environment at Southwark Council, said: “This technology could revolutionise the way we keep our roads safer in Southwark.”

Andrew Howard, head of road safety at the AA, said: “Drivers are divided in their views of intelligent speed adaptation. Some hate it, some want it. Many have questions that will be answered only by trials like those being carried out by TfL.”

TfL said that it had no plans to insist on the device being fitted to any vehicle and hoped that drivers would agree to do so voluntarily