Hundreds of thousands of drivers caught on camera speeding and going through red lights are not being prosecuted, a BBC investigation has learned.
Some drivers register their cars at another address
The figures, for England and Wales, suggest London is particularly bad, with more than half of cases failing.
The BBC Radio 4 probe found cameras in the capital catch about 500,000 people a year, but a third of those cannot be traced to an address.
The Metropolitan Police said tracking down drivers was a national problem.
The Investigation programme found it was difficult to produce national figures partly because there were differences in the way some areas collect their data.
But according to figures supplied by Safety Camera Partnerships to the government, hundreds of thousands are getting away with breaking the law.
Of the 500,000 people caught on camera in London each year one third cannot be traced - either because they are foreign vehicles, or they are not registered with the Driver and Vehicle Licensing Agency (DVLA).
There can also be other problems such as technical faults with cameras and emergency vehicles.
About 350,000 people who have been caught are sent a "Notice of Intent to Prosecute" but only 48% end up getting points on their licences and a fine.
Other parts of the country had also had significant problems, and the programme said it had uncovered evidence drivers were registering cars at addresses other than their own to evade capture.
The Association of Chief Police Officers traffic spokesman, Chief Constable Meredydd Hughes, said detection rates for all crimes was 30% and that in that context speed camera offences were being reasonably enforced.
Kevin Delaney, former head of the Met's traffic police, said the figures were evidence of a wider problem that speed cameras can only catch people who are basically law-abiding.
"Any form of remote detection such as speed cameras relies on the information supplied by the public.
"If that is not correct then remote detection immediately falls flat. You need traffic police to catch the problem drivers."
A Department for Transport spokesman insisted safety cameras played a valuable role.
He said: "Independent research has conclusively proved that cameras save lives, with around 1,745 fewer people killed or seriously injured each year at camera sites.
"A gap between the number of prosecution notices and fixed penalties issued does not mean large numbers of drivers are escaping punishment."
He added: "Some drivers are offered a speed awareness course instead, have their cases heard in court or advise the police that someone else was driving."
Dianne Ferreira, communications officer for road safety charity Brake, said speed cameras were "a proven deterrent".
She added: "It's crucial that the camera system works properly. If many people are evading prosecution then the system needs to be looked at.
"It's important that drivers do not get the message that somehow it's all right to speed. A system that saves lives should not be faulty."
However, campaigner Paul Smith, of Safe Speed, said the system was a "disaster from beginning to end".
Mr Smith said the cameras made road safety "worse" by "misdirecting life-saving resources".
He said: "Enough is enough. Speed cameras are a bad joke. Let's pull the plug. They are at the heart of a failed policy that's costing real lives."
BBC Radio 4's The Investigation will be aired at 2000 GMT on Thursday 19 April