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.
"They mayor now wants us to hit a target of 50% and ISA is one of the ways we are looking at to achieve this."
But the London Assembly Conservatives said if ISA is introduced every car entering London would need to be adapted, potentially at cost to the motorist.
The party's transport spokesman Roger Evans said: "This proposal is ill thought out, expensive and dangerous. We have real fears that accidents may result from car engines cutting out during manoeuvres.
"Drivers must be able to trust their vehicles at all times," he said.
But the TfL spokesman said at the moment they are not being considered for private vehicles.
"We are working on the theory that if you have enough vehicles like buses and taxis on the road travelling at the correct speed, then this will force other vehicles to move at the same speed."
TfL hope to carry out a trial of the scheme, possibly at the beginning of next year, involving 10 vehicles.