The hi-tech police weapon being wielded in drug-driving battle

But police who stopped Scott Greenfield believed he was under the influence and unfit to drive.

Eight minutes later he failed a roadside test and was arrested, part of a process that has led to him being banned from the road for three years.

Greenfield is one of the victims of a revolution in the fight against people who take drugs and then get behind the wheel of a car.

It used to be incredibly difficult to prove someone was under the influence of drugs. It was a subjective decision by a policeman followed by lengthy process to decide whether to go ahead with an arrest.

Now, with science involved, it takes eight minutes.

Greenfield pleaded guilty to driving with a level of a controlled drug above a specified limit when he appeared at Telford Magistrates Court.

Abigail Hall, prosecuting, said that officers on a mobile patrol on June 6 had pulled over Greenfield's van on the B4176 heading towards Bridgnorth because one of the tail lights was not working.

When the driver got out of the Mercedes Sprinter van officers could smell cannabis coming from inside it.

Mrs Hall said Greenfield had subsequently failed the saliva test and was arrested. He later gave a reading of 3.4 microgrammes of THC (the active component of cannabis) in his blood – the legal limit is two microgrammes.

The court was told that Greenfield, of Forest Road, Dudley, had two previous convictions for drink driving in 2008 and 2009 and as a result faced an automatic ban of three years.

He had also previously been fined for using a mobile phone while driving and contravening solid white lines.

As well as the mandatory three-year ban, magistrates fined him £350 plus a total of £270 in court costs.

Mr Robert Bimpson said that on the evening in question Greenfield had been taking a friend back to Bridgnorth and had been offered a joint that the friend had been smoking in the van.

Mr Bimpson said that Greenfield had taken a few drags on the joint while in the van, an action the defendant now called "a matter of abject stupidity".

While police had issued an on the spot fine to his friend for possessing the drug when they stopped the vehicle, Greenfield now faced a lengthy driving ban, he said.

The change in the law meant that drivers could be prosecuted, even if their standard of driving was not called into question, said Mr Bimpson, who added that Greenfield had been pulled over because of a fault with the van, not because of his driving.

"The level has been set deliberately very low," he said and told the court that in some areas 90 per cent of motorists subjected to saliva tests were failing them, compared to 30 per cent for alcohol tests.

He added that the defendant's reading should be put into perspective and that in future people would be coming to court with much higher readings of 10 microgrammes who would be driving while seriously impaired by the drug.

The conviction of Greenfield was today welcomed by Inspector Jeff Morris, of West Mercia Police.

He said it was a case that could have slipped through the net before the introduction of the new tests.

He added the drug testing kit had proved popular with police because they are so easy to use.

He said: "The best way to describe the device is that it looks like a pregnancy test, and if someone tests positive for drugs two lines appear on the screen.

"It detects cannabis and cocaine and a substance called BZE which is found in cocaine.

"It's very simple to use and can pick drugs up in a person's system up to three days after they were taken, which perhaps people don't realise.

"When we pull someone over and suspect they may have taken something we get one of the swipes and it's so simple to use.

"They have to lick the pads inside until it turns yellow, then we press a button which releases chemicals. We hold it up for 10 seconds then lie it down for eight minutes and wait.

"If someone tests positive two lines will appear next to whichever substance they have taken and then we will arrest them.

"Once they are arrested they go back to the station for a blood test and hopefully they will be prosecuted."