A road safety campaigner is calling on the Government to revisit drugged driving laws, following two fatal crashes where the drivers had evidence of cannabis in their blood.
In both crashes, which claimed the lives of three people, the drivers were charged with careless driving causing death.
Rachael Ford of the Candor Trust said the cases exposed the limitations of drug driving laws introduced five years ago.
"Careless driving is in no way comparable to drink driving and drug driving. It's like doing an armed robbery and getting charged with jaywalking."
Most recent Ministry of Transport figures show just one person has been convicted of drug driving causing death.
On Thursday, truck driver Donald Newman was sentenced to 220 hours of community work for careless driving causing the deaths of Margaret Turnbull and her son Trent Whitburn.
In the horror crash near Sanson last January, Newman, 43, drove his truck into the back of Mrs Turnbull's car, forcing it into another truck and causing the vehicles to erupt into flames.
A blood sample later taken at hospital found a cannabis level of 0.8 micrograms per litre - the equivalent to smoking a single joint within 1 to 7.5 hours before the sample being taken.
Newman said he smoked cannabis five days before the crash, and his lawyer Paul Murray said there was no evidence he was impaired by the drug.
He said the likely explanation was that Newman fell asleep.
In the Palmerston District Court, Judge Jennifer Binns said she didn't doubt ''there might be an impact'' from smoking cannabis.
However, there was no evidence before the court of Newman being impaired.
Under drug driving legislation, Police must prove a driver was impaired and a blood test alone is not enough.
Impairment is usually judged by a Compulsory Impairment Test (CIT), which involves putting a driver through a series of balance and co-ordination tests.
But if a driver is taken straight to hospital they cannot be put through a CIT.
Campaigner Ms Ford says the impairment test was ''absolutely useless'' in serious accidents where people were injured.
''And this is where the impairment is going to be, it's going to be in people who are having serious crashes. So it's like the police are blinkered, they're being blindfolded.
They're not able to actually pursue something that's a serious road safety issue.''
Ms Ford urged the Government to bring in drug driving limits like in some states of the USA and Australia.
The UK has set drug driving limits which are due to come into effect next March.
"Our government needs to be looking at legislation to modernise and just drag itself into this century for the wellbeing of people that are using the road, the safety of innocent people."
But Associate Transport Minister Michael Woodhouse says the science is not yet good enough to introduce a drug driving limit.
"We've got really good science around the correlation between the level of alcohol in somebody's system and impairment. We were advised we don't have the same scientific base to accurately correlate the level of THC in somebody's blood system with their impairment."
He concedes the current system is not perfect, and he would support drug driving limits if impairment could be proved on blood alone.
"When the science provides the Police and the Courts with the confidence that a level of THC in someone's system is correlated with impairment, I have no doubt that the law will be changed to make sure that a person in that situation, who is impaired, would be charged with drug driving causing death."
In addition to the community sentence, Newman was disqualified from driving for 14 months, and ordered to pay $6000 to the victims' families.
Tonight Sunday looks at the law around drug driving, and investigates another high profile case in which cannabis was found to have been used by the driver, this time within an hour of the fatal crash. SUNDAY - TVOne, 7pm.