There are concerns the legalisation of cannabis could lead to an increase in road deaths, with the impact of drugged driving already becoming more apparent.

In 2019, alcohol or drugs were a factor in 131 fatal crashes on the road, and those numbers have been rising steadily over the past five years.

While some advocates believe those numbers would increase further if weed was made legal, the limited scientific evidence on offer does not back that up.

Karen Dow's son was killed in a road crash when the other driver was high on cannabis and other synthetic drugs.

Having gone through that heartbreak, she said New Zealand needs to crack down on drugged driving.

"As a country we seriously need to look at zero tolerance," Dow said. "Lives are too precious and it's too easy for people to make a mistake."

Dow said the zero tolerance for drugs should extend to alcohol, and anyone caught with either in their system should feel the full weight of the law.

She said legalising marijuana would only confuse that, leading to more people driving high, and a higher number of road deaths.

"We have to draw a line in the sand and actually say what is acceptable and what is not acceptable behaviour," she said.

"I don't consider any degree of alcohol in your system for driving to be okay, and I think the same needs to be said for drugs.

"We need to have a triage system like they do in the UK, where it's laid our quite plainly."

In Britain, anyone found to have cannabis in their blood or urine while driving faces a minimum one-year driving ban, a fine, up to six months' prison, and the finding goes on the person's criminal record.

'The more you test the more you find' - professor

Professor Joe Boden from the University of Otago is on the cannabis panel assembled by the prime minister's chief science advisor.

He agreed that there should be a zero tolerance for people caught driving while high on weed, and said it should be the same for alcohol.

But he said the evidence does not suggest an increase in road deaths or crashes if cannabis is legalised.

"In Colorado, when cannabis was legalised, they said there was a very large increase in the number of people caught drug driving," Boden said.

"But what actually happened is, previous to legalisation, [the police] had three mobile units and that increased to 15.

"So it's sort of like Covid, the more you test the more you find, if you see what I mean."

Professor Boden said in places like Uruguay, Canada, and states in the US, cannabis usage has not changed much.

Users continue to use, and legalisation is not luring scores of people into becoming drugged drivers.

But he said if the referendum does pass next month, it is vital that a roadside drug testing bill is passed before cannabis is made legal.

That is because the jury is out when it comes to impairment from marijuana use.

"The science behind it isn't remotely settled enough to work out what a safe level of use would be in terms of being behind the wheel of a car.

"Now we're hamstrung by the fact in many areas such as this, and medicinal cannabis and all of those areas, because it's illegal and you can't actually do the studies, giving them the drug and getting them to perform tasks and those sorts of things."

AA wants roadside drug testing system

Dylan Thomsen, the road safety spokesperson for the Automobile Association, said the AA is not taking a side on the referendum.

But he agreed that a roadside drug testing system must be in place before cannabis can be legalised.

"When you're driving it's about being 100 percent focused and alert and mentally clear so anything that you can do or take that is going to alter your mental state is not a smart thing to be doing before you're getting behind the wheel."

Thomsen said the AA has reviewed the latest research from other countries on the effects of cannabis legalisation on driving, which is mixed.

For that reason, the AA is focused on ensuring the testing is in place, rather than supporting or opposing the bill.

See more RNZ coverage of the referendum