According to a recent BBC News article - 'TfL looks at car speed limiters' - London transport is looking into a system that would have control over the speed of vehicles:
'Transport for London (TfL) has said it is working on a system which could automatically reduce a vehicle's speed. But TfL said the Intelligent Speed Adaptation (ISA) scheme is only being considered for public vehicles such as buses and taxis.
A spokesman said at first the scheme will alert drivers, via technology such as a satellite navigation system, that they are moving into a new speed zone.
He added ISA could help in reducing road deaths.
"We have met the government target of reducing casualties on the roads by 40% by 2012," TfL's spokesman said.'