We're not just drinking before we die on the roads - we are drinking more.

More Kiwis are driving drunk all the way to the grave following the lowering of our alcohol limit.

A just-published research paper has shown those who had been drinking, driving, then dying were drinking more than before, and more drugs were thrown into the deadly cocktail.

Forensic toxicologist Hilary Hamnett led the study, which was thought to be the first of its kind, taking a look at alcohol and drugs in dead drivers and motorcyclists.

Hamnett, who previously worked at Institute of Environmental Science and Research (ESR) in New Zealand then at University of Glasgow in Scotland looked at both countries, which lowered their drink-drive limits about the same time.

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New Zealand's law change, lowering the blood alcohol limit from 80 milligrams of alcohol per 100 millilitres of blood to 50mg came into effect in December 2014.

In the year leading up to change there were 30 driver deaths with positive alcohol readings, while the following year there were 48, according to her research.

As a proportion of all driver fatalities, those with alcohol involved rose from 23 per cent in the year before the change to 32 per cent in the year after. The number of fatalities involving people who were over the drink-drive limit at the time also increased in 2015.

Passengers, cyclists and pedestrians who were killed in crashes involving alcohol were not counted in the study.

ANDY JACKSON/STUFF The impact of alcohol on fatal accidents seems to be worsening after a lowering of the drink-drive limit.

What Hamnett found was that New Zealand drivers who died on the roads were more likely to have been drinking after the law change, those that had been drinking had higher blood-alcohol readings and were more likely to also have drugs in their systems.

The study, published in the Journal of Forensic Sciences, did not speculate on why the law change would have such a counter-intuitive result but, on the phone from United Kingdom, Hamnett said it was "an interesting effect".

Broader social factors may have played a part, she said.

High-profile campaigns about the lowered limit ran in both countries but there was little evidence showing they had worked.

The one glimmer of hope in the study was seemingly unrelated the the lowered limit. Those who tested positive for drugs – but no alcohol – dropped after the law change. But if alcohol was also involved, the chances of drugs also being there increased.

Cannabis was the most-popular drug, followed by prescription drugs, over the counter drugs, then opioids.

Acting road police national manager Virginia Welch said the findings had to be seen in context of other factors such as more road use, "exposure to higher-risk rural roads", and an overall increase in fatal accidents.

"As a proportion of all fatal crashes substance-involved fatal crashes have remained about the same," she said.

"There are many factors that contribute to [a] crash and many factors that lead to a fatal injury resulting. It would be naive to pick one out of the lot, while leaving all else to the side, and attributing to it a change in crash numbers."

New Zealand Initiative's Sam Warburton said the small numbers in the survey and other factors,such as changing levels of enforcement, made it hard to say how much weight to give the findings.

The lowered drink-drive limit was "never going to be a silver bullet" and the Ministry of Transport had estimated it would only result in three-fewer deaths per year. That was an "amount impossible to detect in data", he said.

Road safety campaigner Clive Matthew-Wilson,editor of car review website Dog and Lemon, said the findings were unsurprising.

"Lowering the drink-drive limit without restricting access to alcohol is like telling kids not to eat candy, then leaving them outside a candy store."

Associate Transport Minister Julie Anne Genter wanted to give officials time to look at the study before commenting.

* An earlier version of this story incorrectly reported the percentages of road deaths involving alcohol in the years before and after the blood-alcohol limit was lowered.

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