It works like this. You're speeding along the motorway, one eye on the mirror, when a police car signals you over to the hard shoulder. Out steps the officer, radio blaring, and asks you to blow into his breathalyser.
Of course the crystals don't change colour. You're not stupid enough to drink and drive. You take the "clean and safe" option, and celebrate your success on the trading floor by chopping out a little coke. But this is your unlucky day.
He may not have any sniffer dogs, but if there's any cocaine in your bloodstream the officer will find it, using a new gadget devised by government scientists, and likely to lead to a surge in prosecutions.
Motorists have grown used to the annual campaign warning them not to mix alcohol with the road. Now the same combination of testing and advertising is to be aimed at combatting the rise in cocaine abuse.
Keith Hellawell, the drugs czar, is working on the campaign with a panel of government experts to target successful, middle class achievers who are the fastest growing group of illegal drug users.
This new generation is drawn to cocaine in the misguided belief that it is purer and safer than other hard drugs, and that driving under its influence is less risky than getting tanked up on booze. But statistics show that 20 per cent of those killed on roads in Britain over the last year had illegal drugs in their bodies.
"My children are critical of my generation for drink-driving," said Mr Hellawell, "but drug-driving is just as grave. Its effects can be catastrophic."