Less than three quarters of fixed speed cameras around Norfolk are actually switched on and catching offenders, it has been revealed.
New figures show 17 of the county's 24 police-operated fixed speed cameras - or 71pc - were in operation.
In contrast, data released after an Freedom of Information request shows that 100pc of Suffolk's cameras are working - but there are only four across the entire county.
And the figures for Norfolk are still well above the national average.
Just 52pc - 1,486 - of the 2,838 cameras in the 36 police forces across the country which did respond are switched on.
And four police forces - Cleveland, Durham, North Yorkshire and Northamptonshire - admitted that none of their fixed speed cameras were active.
Cambridgeshire Police did not respond to the request, conducted by Press Association.
Road safety charity Brake described the figures as concerning and called for all cameras to be switched on, while AA president Edmund King said the high number of inactive cameras was down to pressure on budgets.
A spokeswoman for the National Police Chiefs' Council said the decision to use cameras was 'an operational matter', adding that 'all forces have individual responsibility for their use of speed cameras'.
Mr King said: 'Many of the empty yellow cases are due to cuts in road safety grants and the fact that digital cameras, although more effective, are very expensive.
'It is also reflective of the fact that proceeds from cameras are no longer allowed to be ring-fenced to be reinvested into yet more cameras as now all the money goes to the Treasury.'
He warned motorists against gambling on a camera being inactive.
He said: 'Drivers who play Russian roulette with fixed-site speed cameras are playing a dangerous game. Our advice is stick to the limits rather than gambling on the yellow boxes.'
Do they even believe in cameras?
But Claire Armstrong, co-founder of lobby group Safe Speed, which campaigns for more traffic police officers rather than speed cameras, said the investigation 'proves police forces don't believe in cameras'.
She said: 'Forces are conning the public into thinking cameras are there for road safety because, if they really thought that, every single one of them would be on.
'They are a flawed road safety policy and the only way to truly improve that is with more traffic police officers on the roads.'
She added: 'I am glad there are only 52% working - and we'd actually like to see less.'
Neil Greig, spokesman for the charity IAM RoadSmart, which campaigns to make roads safer by improving driver and rider skills, said it believed all speed cameras should be active.
He said: 'Drivers should be in no doubt that every yellow box they pass is active and police forces and safety camera partnerships should all be aiming for their cameras to be vigilant 100% of the time.'
And Jason Wakeford, director of campaigns for road safety charity Brake, said: 'A staggering 1,800 people lost their lives on British roads last year and speeding is a factor in thousands of crashes.
'Speed cameras are a proven, cost-effective way of reducing deadly collisions and so it's critical they are operational.
'We are concerned to see figures which suggest so many are switched off and would urge they are urgently put back into action.'
Steve Gooding, director of the RAC Foundation, said: 'It would be a dangerous and foolish driver who was tempted to speed because they thought they'd get away with it.
'Not only are cameras routinely moved between housings, police also use mobile cameras, while several hundred miles of British roads are now being monitored by average speed check systems.
'The majority of the public back the use of speed cameras, but the location and effectiveness of the devices should be the subject of regular scrutiny.'
The figures cover all police fixed speed cameras, but not the mobile devices forces also use.
The Department for Transport was contacted for comment.
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