Only about half of fixed speed cameras in the UK are actually switched on and catching offenders, figures obtained by the Press Association indicate.
Data released by 36 of the 45 police forces in the UK found that four have no fixed speed cameras at all and 13 have fewer than half actively catching speeding drivers.
The Press Association sent a Freedom of Information request to all 45 forces and their speed camera partnerships asking how many fixed speed cameras they have and how many are active.
The 36 which responded with data had a total 2,838 cameras, of which 1,486 (52%) were active. Nine refused to disclose the information or failed to respond.
The figures cover all police fixed speed cameras, but not the mobile devices forces also use.
All forces and speed camera partnerships that responded said they deployed regular mobile speed cameras across their areas. They also said they regularly review which fixed cameras are turned on.
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.
The forces which said none of their fixed speed cameras were active were Cleveland, Durham, North Yorkshire and Northamptonshire.
The Northamptonshire force said it turned its cameras off in April 2011, but has left the structures in place to deter speeders.
Staffordshire Police have 272 fixed cameras across their patch, but just 14 of them are active, while the Derbyshire force operates 112 cameras with just 10 of them catching speeders.
The forces with a quarter or less switched on are West Yorkshire (25%), Kent (25%), South Yorkshire (24%), Greater Manchester (24%) and Cheshire (17%).
City of London, the Metropolitan Police/Transport for London, Lancashire, Nottinghamshire, Suffolk and Northern Ireland said all their fixed cameras were active.
West Midlands Police, who announced in 2013 that their old speed cameras were being switched off, said they now had eight new average speed cameras.
Figures released last month showed that more than 500 speeding tickets a month were clocked up at one hotspot in Birmingham.
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.”
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.”