It's not news that Britain has a lot of surveillance cameras, with around 60,000 run by local authorities alone. However, most cameras record only images, which are normally kept for a few weeks. Unless and until facial recognition technology improves significantly, these are not capable of creating a database of people's movements.

But the police's network of automatic numberplate recognition (ANPR) cameras – which now total 10,502 in England and Wales – already do just that. The cameras aim to read the numberplate of every passing vehicle, with the data held for two years in the National ANPR Data Centre, regardless of whether the vehicle is linked to crime. (A move to keeping it for five years appears to have been shelved, following discussions with the information commissioner's office.)

Furthermore, the locations of most of the cameras, including some run by local authorities on behalf of the police, are secret. This is unnecessary: normal CCTV cameras are accompanied by signs disclosing the responsible organisation, and speed cameras, which are on roadmaps, are signed and painted yellow.

Police ANPR has not attracted as much attention as other types of surveillance, except when its use has been controversial, such as stopping those who are innocent of crime but have been placed on a police watchlist. Forces seem to take their own approaches. Devon and Cornwall has disclosed that it uses ANPR in Torquay, Brixham and Dawlish, without giving precise locations or spending figures. Thames Valley police, which has released details of spending but not locations, has put nearly £2m into 47 fixed cameras, 31 in road vehicles, 11 portable kits and one in a helicopter.

Police argue that revealing camera locations would allow criminals to drive around them. But that assumes that such villains lack reconnaissance abilities: ANPR equipment can be easy to spot for those who know what they are looking for. Most of the cameras on motorways and A roads belong to the government's Highways Agency (and its equivalents in Scotland and Wales) and private company Trafficmaster. Both use special ANPR cameras that derive non-unique codes from passing numberplates. Their central systems then look for the same code further down the road, to calculate average speeds for traffic information purposes. Also, they usually monitor just one lane.

Observations suggest that the 100-mile stretch of the M5 between Cheltenham and Exeter has lots of single cameras, but just one location with cameras for each lane, which are of a different shape and colour to those used by the Highways Agency and Trafficmaster. A similar pattern can be seen on other motorways.

If there are relatively few police ANPR cameras on major roads and in forces such as Thames Valley, where are the bulk of the 10,502? An educated guess would be big cities, particularly London. But the secrecy makes it very difficult for the public to evaluate the worth of the technology.

Police forces should end the mystery of their fixed ANPR camera locations. If these were published, rings of fixed cameras – such as that which covers every entrance to the centre of London – would still be effective, as they should catch all traffic entering the zone, and could in fact have a greater deterrent effect. It would not affect mobile use of ANPR, and overall the use of the technology would become more accountable. And if disclosure of location would make some standalone fixed cameras useless, it suggests that their efficacy is already dubious, and will decline as more people realise where they are. Finally, it would mean the police were no longer using secret cameras to record the movements of millions of the people who pay their wages.