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

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 beltway, which surrounds the city.

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.

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