Bright high-adhesive stickers costing a few pence could help reduce crime, which is costing Britain almost L1 billion a year.
While car makers spend millions of pounds installing locking systems to try to deter thieves, police forces believe a scheme requiring motorists' co-operation and a couple of securely stuck fluorescent discs may cut substantially the number of cars stolen.
About one in five of the 9,443 vehicles stolen in South Yorkshire last year were taken in the five hours after midnight. Patrol car officers will know that they are likely to be tracking thieves when they see a moving car displaying fluorescent discs during the agreed 'curfew' hours.
The idea, originally from the United States, has been tried successfully for nine months in Cumbria, and a Metropolitan police pilot scheme is running in north London.
Of 5,000 cars registered under the scheme in Cumbria, only three have been stolen. Two were recovered and the third was stopped by police. No thefts have been recorded from cars displaying the fluorescent discs.
This has convinced South Yorkshire police that the scheme could work in a region with more crime than rural Cumbria. They have an initial target of 10,000 vehicles but believe thousands more motorists will join.
Chief Inspector John Heritage, of the force's crime prevention department, said: 'it is a very simple idea. If we spot a car on the road in the hours when the motorist has said it will not be driven, it can be checked unhindered immediately. The bonus is that the thieves also know that the car will be stopped on sight and checked.'