Alcohol breath-testing of drivers at checkpoints is a useful arrow in the quiver of measures to limit the road toll. The chance of being caught will deter some, even if not all, from driving under the influence.

But other drugs, too, impair the ability to safely handle a motor vehicle. They are also a significant cause of motor crashes, of injury and death.

Effective testing for them, however, is technologically and legislatively more difficult - at least at present.

National MP Nick Smith has launched a petition from Nelson calling for the random introduction of testing. The Government itself says it is some way down the track with this. There appears to be the will. If only it was straightforward.

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For years, National MPs themselves seemed to kick the issue down the road. Now, it is Labour's turn to do the same. It is always easier to have the answers in Opposition.

The Institute of Environmental Science and Research carries out drug and alcohol tests for the police, Ministry of Justice and NZ Transport Agency. It warns the law would need changing and a consensus required on how screening would be carried out before roadside tests for drugs could go ahead.

The agency's forensic toxicology and pharmaceutical manager, Mary Jane McCarthy, has said, as well, it was not known what type of technology might be used in random testing and it was difficult to say when this could happen. She noted limitations to saliva tests, used overseas.

Blood tests would be best. But taking blood or urine samples routinely would be impractical and invasive. Alcohol, on the other hand, is quick and easy to screen with a breath test. The full blood test is in the background as a follow-up.

It is already an offence to drive with evidence in the bloodstream of a qualifying drug. But the presence of the drug is not enough, as the Ministry of Transport has explained. There has to be impairment, and that needs to be demonstrated by unsatisfactory performance of the compulsory impairment test.

What happens is a police officer first needs "good cause to suspect" drugs have been consumed. That cause might be erratic driving, or, crucially, someone at an alcohol checkpoint might pass a breath test but still behave as though intoxicated.

The officer then can make the driver take the test, which includes an eye assessment, a walk-and-turn assessment and a one-leg stand test.

Should the person fail, the officer can then forbid the driver from driving and may require a blood test. About 90 per cent of those taking the blood test are subsequently convicted. But the number is only about 400 a year, compared to about 30,000 alcohol driving convictions.

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The AA is pushing for saliva tests, while the New Zealand Drug Foundation is cautious because of inaccuracies, believing the testing is expensive and arduous and the research showed it just was not there yet.

A key issue is to catch a level of likely impairment, not simply some trace of drugs. Cannabis, for example, can remain in small levels in people's systems for a long time, long after it has had any significant effect.

The legalisation of medicinal cannabis and the possibility of loosened recreational use will raise further questions about the need for roadside random tests.

New Zealand could follow overseas models and plunge ahead with random saliva tests, despite their limitations and their potential unfairness and injustice. Or it could wait as technology rapidly develops for quicker and more effective tests.

It could well be New Zealand cannot wait for the better technology because of the damage drugged drivers cause. But this extremely serious matter needs careful consideration and debate - not just emotive responses to the horror of road crashes and the involvement of drugs.

- Otago Daily Times