Drug charities have broadly welcomed new powers for police to carry out roadside impairment tests on drivers they suspect have been taking drugs. But how effective are the tests, which include co-ordination challenges, in identifying drug drivers?
 DrugScope, Release and Transform all support the tests |
Regular cannabis smoker Michael, 33, from north London, says he has driven under the influence of the drug on many occasions.
He said: "It is very obvious to me that alcohol impairs my senses.
"I don't need to get into a car to know that so I don't drink and drive.
"But I have no problem with cannabis as I feel perfectly in control."
 |  The only way a policeman would pull me over while I was under the influence of cannabis would be as a result of a random stop |
He is confident he would not get caught drug driving after smoking cannabis because, he says, the only way he would be pulled over by police would be as a result of a random stop because "there's no way a policeman would know otherwise".
Michael believes he would pass a roadside impairment test because he says he has built up a tolerance to the drug and is therefore able to function normally after smoking it.
He thinks there are thousands of other cannabis users like him who will never get caught by the testing system.
'Probably impaired'
A spokeswoman for the charity DrugScope, which welcomed the tests, said they were in place precisely to assess whether the judgement of people like Michael had been impaired by drugs.
| DRUG DRIVE TESTS The five tests police use to check if a driver is under the influence 
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She said: "In a scenario like Michael's he probably would be impaired.
"If it's immediately after someone's taken drugs then they will be effected in some way."
DrugScope, like most other drug charities, supports the roadside tests specifically because they test impairment rather than just the presence of drugs in the blood stream.
"If someone tests positive for drugs in their bloodstream it doesn't mean they are under the influence of the drug," said the spokeswoman.
"Cannabis, for example, stays in the bloodstream for weeks," she said.
But the "bottom line", she added, was that "if someone can't pass this test they're not fit to be driving".
Competence increased?
Transform Drug Policy Foundation spokesman Steve Rolles said it also supported the impairment tests, as opposed to blood or urine tests, as "a sensible way forward".
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But he suggested some drugs may even increase driving competence.
He claimed the RAC was marketing its own caffeine-based stimulant.
Mr Rolles said: "We've got the situation where the RAC is marketing a stimulant drug, albeit a legal one.
"So, presumably other stimulants, like amphetamines, may potentially also increase competence."
Gary Sutton, head of drugs services at Release, said someone under the influence of amphetamines might fail the impairment test at the first hurdle - the check for dilated pupils.
| Across the board, if the police stop someone and they can't do, for example, a four times table it's fairly clear that they're motor neuron-impaired behind a wheel |
But were it not for that, it was possible they could give "a relatively decent performance", he said.
He agreed with the DrugScope spokeswoman that "across the board, if the police stop someone and they can't do, for example, a four times table it's fairly clear that they're motor neuron-impaired behind a wheel."
A spokesman for the Association of Chief Police Officers said the tests had already proved to be "very effective" in identifying drug drivers.
'Back to station'
Before Wednesday, drivers could choose to take part in the tests but refusal is now an offence in the same way as failure to provide a breath test.
Once a driver had been stopped on suspicion of "driving under the influence", officers would assess whether they had been drinking alcohol or taking drugs, the spokesman said.
| It's important to use the impairment tests as an indication before going back to the station to be sure |
If, for example, the driver had dilated pupils and officers suspected they had taken drugs, the impairment test would be carried out.
"It's important to use the impairment tests as an indication before going back to the station to be sure," he added.
He dismissed the notion that people could take illicit drugs without impairing their ability to drive and said those driving while under the influence of drugs were likely to be caught.
"At the end of the day, police are trained in this and if they suspect someone is under the influence they will be stopped."
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