Enforcing the law on drug driving has not been a problem historically of either political will, or policing priorities. The main issue has always been applying the science and technology to make the law work.

This week’s announcement of a “crackdown” on the “blight” of drug driving has all the hallmarks of a New Labour announcement: a story spun over a whole weekend presenting both the UK government’s tough and humane sides. Road safety minister Mike Penning, from the comfort of the breakfast TV sofa, said soon there would be drug limits like there are with alcohol. He even offered the idle threat: “If you drive while under the influence of drugs, you will not get away with it.”

Yet it would appear this announcement is somewhat premature; prescribed limits for controlled drugs, like we have had for alcohol since 1967, appear as far away as ever. The Labour government consulted on proposals for drug driving in November 2008; little progress appears to have been made since because of intractable technological problems.

Many questions remain unanswered. How do you set a specific limit for a drug when the scientific measure of the effect of drugs lacks research, where purity can be extremely variable, and when there are so many drugs available, both legal and illegal? Alcohol’s effects on physiology and impairment can be tested quite precisely with willing subjects and driving simulators. There would be some tough ethical issues to contend with, no doubt, over similar Transport Research Laboratory trials with crack cocaine and heroin.

The European Monitoring Centre for Drugs and Drug Addiction’s 2009 report “Drug Use, Impaired Driving and Traffic Accidents” said:  “It is difficult to apply the BAC (blood alcohol concentration) parallel to other psychoactive substances because of the vastly different natures of the range of substances involved, the limitations of experimental and epidemiological research in trying to determine a cut-off level, the ethical considerations involved in its enforcement and the question of combining or separating drug abuse and road safety measures.”

There are other significant scientific problems with testing drivers for cannabis. It is easily the most prevalent drug in Britain; the British Crime Survey 2010/11 showed 2.2 million 16-59 year olds took cannabis in the last 12 months. Opiates and stimulants tend to pass through the system in about a day; however cannabis is absorbed into the muscle fat and stays in the body for about a month, long after the driver has ceased to be intoxicated by its effects. So to set a specific limit for cannabis would equate to criminalising its use for those who drive, irrespective of the road safety benefits.

In spite of the government flexing its legislative muscles, there is still a very long way to go before this new specific offence can sit alongside that of drink driving. By the government’s own admission, the screening devices to be used at the roadside have not yet completed the rigorous type approval process. The evidential device to be used at the police station with those prescribed limits is still some way behind. The panel of health experts who will be setting prescribed limits for legal and illegal drugs has not yet convened and the list of drugs they will be assessing has yet to be agreed. The only things so far in place are the draft crime, communications and court bill in the Queen’s Speech and the Department for Transport’s press release promising tough action.

Danny Kushlick, Head of External Affairs for the organisation Transform, said: “The danger is this becomes a disproportionate exercise in net widening, in so far as it will unnecessarily criminalise drug users, when their drug use is detected, in the absence of impairment and danger. Much of the political rhetoric on drug driving is little more than propaganda, intended to show that government is protecting us. If the government was serious about reducing dangers on our roads, it would implement testing that effectively identifies impairment, rather than evidence of drug use.”

Drug driving is a significant problem. One survey showed 18% of road fatalities had controlled drugs in their bodies; three-quarters of the samples were for cannabis. Even if one excluded all the positive tests for cannabis, that would still amount to about 130 deaths a year from crashes involving other illegal drugs. But this much trailed new law will not reduce that figure.

The relevant section of the Road Traffic Act 1988 of being “unfit to drive through drink or drugs” derives from legislation dating back to the 1920s. The penalty is identical to what is proposed for the “new” offence: a maximum fine of £5,000, 6 months imprisonment and an automatic 12 months disqualification.

Ellen Booth, senior campaigns officer for road safety organisation Brake, took a more positive view: “This is an incredibly important step forwards in tackling drug driving which Brake welcomes wholeheartedly. Creating a new offence as well as approving roadside drug screening devices by the end of 2012 will make an enormous difference in preventing drug driving crashes, and also ensuring justice for families whose lives are turned upside down by selfish drug drivers.”

The new screening devices will be a welcome development but it could take some years to devise a new evidential machine. Until such time, anyone failing a roadside test will still have to be examined for signs of impairment by a police doctor known as a forensic medical examiner. If satisfied, the FME would then authorise a blood test to be taken which would then be sent off for analysis before a decision, some weeks later, on whether to prosecute. That’s old technology, by any measure.

It is hard to see what is significant about by this announcement – it appears to be simply grandstanding of policy which deflects the public’s attention momentarily from other weightier matters.

Jeremy Sare is parliamentary advisor to the Angelus Foundation