In 2014, there were 50 deaths on NSW roads that involved a driver or rider with an illegal drug in their system, he said. Jimmy Singh, a senior managing lawyer from Sydney Criminal Lawyers, said he had seen "an exponential increase" in drug-driving cases in the past few months and expected they would soon surpass drink-driving caseloads. "Sometimes we are getting three to four clients per week. I won't be surprised if it increases further." But whereas roadside breath testing for alcohol is based on concentration thresholds – under 0.05 is the legal limit for most drivers in NSW – drug-driving tests only screen for the presence of the cannabis, speed or ecstasy in any concentration. Those who return a positive roadside drug test are typically charged under s111 of the Road Transport Act (2013), which prohibits the presence of an "illicit drug in person's oral fluid, blood or urine".

Mr Singh said the rigidity of the current legislation meant roadside drug testing was primarily about criminalising drug use rather than promoting road safety. "It does not appear to have the purpose of protecting the other road users. All they need to prove the offence is a drug certificate that shows drugs were in your system," Mr Singh said. "There is no evidence at all that the drug test they conduct for the s111 offence is actually [testing] for impaired driving. "The system needs to be reviewed to make it more similar to drink-driving." Deputy Premier Grant confirmed the law was underpinned by a prohibition on illegal drug use, saying via a spokesperson: "if anyone is concerned about getting caught, they shouldn't take illegal drugs.

"Drugs are illegal and we've had a horror year on our roads in which drug driving was a clear contributing factor to some horrible deaths." Among those being caught in the law's dragnet are those found to have traces of illegal drugs in their system long after they were consumed, said John Sutton, managing partner at Armstrong Legal, which handles about 15 cases of drug driving each week. "The classic example is cannabis because it stays in your system for a very long time, but the active component – the THC – goes out of your system easily within 24 hours." But the idea a driver could test positive to cannabis days after smoking it was based on a misunderstanding of how the oral test works, said by Professor Jan Copeland, director of National Cannabis Prevention and Information Centre. Roadside drug testing, which takes the form of an oral swab, does not test for metabolised cannabis, which can stay in the system for up to 90 days for regular users.

"They only test for the active THC. While there can be a delay of hours since the person smoked, they can still have active THC in their blood and be impaired. "So the idea that you can be picked up on an oral fluid swab and not be impaired is very unlikely." But whether the law should be amended to include low permitted levels of intoxication, as is the case with alcohol, and as is permitted in some European jurisdictions was a matter for community discussion, she said. "If the drug is made legal under certain circumstances, such as medicinal use, that will certainly bring in a need for this kind of discussion." Of the 2600 people found guilty of drug driving in 2015, almost 70 per cent were convicted and fined (up to $1100 for first-time offenders), resulting in an automatic licence disqualification of six months. A further 700 people were given a good behaviour bond without a conviction recorded, while just four cases resulted in a not guilty verdict.

"It's a bit unfair. A person who is not actually a danger to other road users is now going to potentially face an automatic disqualification of six months and a criminal record," Mr Singh said.