On March 2nd the Department for Transport launched what it calls the biggest shake-up of Britain’s drug-driving laws for 85 years.

That ought to be uncontroversial enough, since it is estimated that drivers under the influence of drugs are responsible for at least 200 deaths a year. And it has always been an offence to drive while impaired by any cause – even before the numerous Road Traffic Acts which spell it out, it could have been classed as criminal negligence. That’s not just drink and illegal drugs, but prescription medicines, tiredness, uncorrected vision or anything else which makes you unfit.

Yet one of the UK’s largest police forces, Greater Manchester Police, announced this week that they do not intend to attempt to enforce the new rules, because they think that there are still a number of serious problems with them.

The basic change which has been made to the law is that the government has set limits for named drugs; their website says there are 16, and then goes on to list 17, which rather bolsters the GMP’s point that there is some confusion over the new regulations.

Eight of the drugs listed are illegal, and the limit is essentially zero for all of them. The others are medicinal (they are amphetamine, clonazepam, diazepam, flunitrazepam, lorazepam, methadone, morphine, oxazepam and temazepam), and varying limits apply.

The other change is that a roadside saliva test can now be administered by police officers who suspect a driver of being impaired. If it shows a positive result, the driver will be taken to a police station to provide a blood sample.

The reliability of the equipment has, of course, been questioned by some sceptics, and some police forces, including Manchester, have complained that they have not yet been trained properly in their use.

But the most obvious limitation of the roadside test is that it will, in any case, only detect the presence of cocaine or cannabis. Even if it works absolutely reliably, it will give no further evidential or legal help to a police officer who suspects a driver might be under the influence of, say, LSD.

The aim of the new rules was to make it easier to convict drivers clearly unfit to be behind the wheel, but several doctors and scientists have pointed out that drug levels are much harder to determine, and to set, than those for alcohol in the blood.

The effective zero limit for illicit drugs should mean that it would be easier to secure a conviction on an obviously incapable driver, if a blood test shows that he has ingested ketamine, MDMA or heroin. But it won’t be picked up by the saliva test.

On the other hand, the drugs which it will pick up can take a while to leave the system, and the rate at which they do so varies greatly from individual to individual. There are recorded cases of heavy cannabis users who quit, for example, but still had traces of the drug in their system six months later.

Both cocaine and cannabis would probably be detectable for several days after one-off use – so a single joint in Amsterdam or Colorado (where consumption is legal) three days ago could potentially lead to a driving ban in Britain and a criminal record. A drug driving conviction, ironically, might also make it difficult to visit America, where marijuana is legal in several states – Washington DC legalised it just this week.

Most attention, however, has been focused on the possibility that prescription medicines could lead to people being convicted. It’s worth remembering again, however, that if you are clearly unfit to drive for any reason you are committing an offence by getting behind the wheel. So if you are zonked on a prescription medicine not specified in the new rules – and there are a huge number of unlisted drugs which warn that drowsiness is a possible side effect – you could still be arrested.

But if someone on a regular medication is above the new limits, even if they are a long-term user who does not feel at all impaired, and has followed their doctor or pharmacist’s advice to the letter, it could mean that they were assumed to be unfit. The police would have physical evidence of a level of pharmaceuticals which the legislation presumes unsafe, and the driver might have to prove that he or she was fit to drive, rather than the burden of proof operating the other way around.

This is problematic, not least because levels in the blood do not directly relate to dosage. Of course, the government has maintained that anyone with genuine medical conditions, and who is fit to drive, won’t be caught out by these new rules. But doubts have been raised by both lawyers and doctors.

And it’s possible that in some of those cases, the levels laid out by the legislation could lead to protracted, difficult and expensive legal battles. That’s exactly the opposite outcome from the original, admirable, objective of making it easier to convicted obviously incapable drivers – which may be why Manchester police are cautious.

Anyone who uses a prescription drug on the list should probably check their dosage and, if necessary, seek advice. But doctors stress that it is as important not to stop taking any drug you have been prescribed without medical advice, as it is not to take anything without a doctor’s advice.

Recreational users of illegal drugs should be aware that they’re already breaking the law, but also that it is now much easier to convict them of drug driving even after they’ve sobered up (or think that they have). It ought to go without saying that no one who feels impaired by any drug, illegal or prescribed, whether it’s on the list or not, should get behind the wheel in the first place. But for safety’s sake we’ll say it anyway.