The French authorities unveiled their new secret weapon against speeding drivers on Thursday but the chances are we will never get to see it.

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A new generation of mobile speed cameras are to be installed in unmarked police cars as the government steps up its fight against speeding drivers.

Unlike fixed roadside speed cameras, the new mobile cameras will be practically invisible to motorists and virtually undetectable on any GPS device or smart phone application.

And motorists are unlikely to know they have even been caught as the cameras can photograph a vehicle without the need to flash.

The cameras will be fixed on the dashboard with a radar hidden behind the number plate.

Pierre Chasseray, head of French drivers organisation 40 millions d'automobilistes told The Local he was concerned the new device would be used to "trap" ordinary road users.

"If these devices are used to trap the real speeding drivers, the criminals and the real dangerous drivers then that will be a good thing," Chasseray said. "If they are used to trap ordinary drivers ,who might be slightly over the limit then that will be counter productive in terms of road safety."

According to French police speed cameras or radars have helped cut average speeds in France by 10kph since 2003. 

“This represents a drop of around 45 to 50 percent in the number of deaths caused by speeding,” said Aurélien Wattez, head of the police’s road safety department.  But certain people have not adjusted their speeds as much as most road users and it’s those people that we are targeting with these new cameras.

In 2012 speeding was given as the cause of around 1000 deaths on French roads, around 26 percent of the overall number of fatalities.

“There are a minority of people who only respect the limitations where there are fixed speed cameras. This system is there to remind them that they have to respect the speed limits everywhere,” Wattez said.

Although the cameras will be in unmarked cars officers will have to wear their police uniforms, which might be the only signal that could give away their presence on the motorway. Drivers rushing down to the south of France this summer should also be careful who they overtake as for the first few months police will only penalize cars who pass their own vehicle at top speed.

The new cameras will be rolled out in twenty departments across the country on March 15th and will only be used to catch those who break the speed limit by a big margin.

The devices are designed to be able to take accurate photos from inside moving vehicles and will gradually replace the older generation of radars, first deployed in 2004.

Any speeds registered by the cameras will be reduced because of a 10 percent margin of error so anyone driving on a French autoroute, where the limit is 130kph, will have to be driving at 146kph to be snared by the new radars.