How does a Roadside Drug Test work? Impairment or Presence?

Recently across Northern NSW there has been an increased police presence and drug/alcohol testing on the off ramps of the M1.

According to BOCSAR, 0.78% of the Northern Rivers population has been convicted of drug driving offences. This is the highest drug driving concentration in the state.

Drug and Alcohol testing is something all motorists who are engaging in the privilege of driving must comply with when asked by a police officer. Failing to comply with drug/alcohol testing whilst driving, just like refusing one at work (generally), is an offence.

Section 110 of the Road Transport Act 2013 (NSW) outlines the various alcohol limits (from BrAC 0.000 to 0.049 depending on the class of vehicle and type of licence).

Section 111 of the Road Transport Act 2013 (NSW) outlines the restrictions of driving with drugs in your system.

Section 111(1) makes it an offence to drive with Cocaine, THC, Methylamphetamine or MDMA in your oral fluid, blood and/or urine.

Section 111(3) makes it an offence to drive with Morphine present in your blood and/or urine.

Cannabis (THC) is legal with a prescription now. Can I drive?

No. Whilst cannabis was moved from Schedule 9 to Schedule 8 of the SUSMP in 2018 (making it something legal to obtain, possess and use with a prescription) the law has not been amended so it is still an offence to drive with THC in your oral fluid, blood and/or urine. If you have an issue with this, yelling "ACAB" past an RDT setup is not actually an effective solution, contacting your local Member of Parliament is. Police sadly do not get to choose what laws to enforce.

Influence v Presence

A lot of the time people argue police are testing for 'traces' (presence) of a drug rather than if people are under the influence. I cannot find a definition for 'influence' under the act or through Law Access NSW's dictionary. So, we have to settle for the Oxford dictionary definition of 'Influence' - the capacity to have an effect on the character, development, or behaviour of someone or something, or the effect itself.

Now what is influence? I would argue with the dictionary definition it is when you are obviously impaired by drugs or alcohol. The law sets limits for alcohol and drugs. For alcohol, here are the limits;

Learner / P1 / P2 Licence

Unrestricted Licence

Driving a car/riding a motorcycle

0.000 grams of alcohol in 210 Litres of breath

0.049 grams of alcohol in 210 litres of breath.

Driving a vehicle that has a GVM >13.9 tonne or that is carrying dangerous goods

0.000 grams of alcohol in 210 litres of breath

0.019 grams of alcohol in 210 litres of breath.

Driving a taxi, bus, private ambulance etc.


0.019 grams of alcohol in 210 litres of breath

Is a person impaired at these levels? Maybe, maybe not. If we take the low range drink driving BrAC - 0.050, some people may be impaired under this cut-off level and some people may not be impaired until after this cut-off level. This is because every person is different, but it is too complicated to have different laws for every person, so they (government) set a cut-off level for alcohol and if you exceed it, you are guilty of an offence regardless of apparent impairment. Alcohol has a scientific cut-off. We accept that, but drugs don't. Do they?

Well, they do. The cut-off for drugs (as found in oral fluid) in a laboratory setting as outlined in Australian / New Zealand Standard 4760:2019 (Procedure for specimen collection and the detection and quantification of drugs in oral fluid) is as follows;


Cut-Off (ng/mL)



















Ok, there is a cut off - but RBT gives me a number, RDT just gives me a line on a stick, how can they prove that is not just it being in there at 1 ng/mL?

It is worth noting, no field test is a final result. With both RBT and RDT a roadside 'positive' still means you need to go to the police station or 'drug bus' to have a second sample taken. In the case of an RBT this is through a Draeger which is calibrated so that a Breath Analysis Certificate can be issued for the courts. In the case of an RDT, if you fail roadside a second sample will be taken for laboratory analysis and your privilege to drive will be suspended for 24 hours.

So, rest assured they will be going to get a laboratory test. Courts will generally not accept field tests as proof a drug was in your system (unless you run off and refuse to provide a second sample, in which case you have failed to provide a specimen which means you are treated as positive).

Field testing kits have a higher cut-off limit than laboratory. This is to ensure that false positives do not occur that often. Police commonly use the Securetec Drugwipe II for roadside drug testing. This only tests for Methylamphetamine (MET) and Cannabis (THC). Securetec publish their cut off limits as (S 604 G and S 602 G are both given as it is unclear which panel is present in the Drugwipe II);

Securetec S 604 G

Securetec S 602 G

Methylamphetamine (MET)

80 ng / mL

100 ng / mL

Δ9-THC (THC/Cannabis)

10 ng / mL

20 ng / mL

What does this mean for roadside drug testing? If you test positive roadside, for Methylamphetamines you are likely at least 3.2 to 4 times over the cut-off limit. For THC you are likely at least 2 - 4 times over the cut-off limit.

No one would argue 'impairment' if a roadside breath test wasn't showing you as positive until a BrAC of 0.160 to 0.200 (3.2 to 4 times the limit).

So before you go and harass some police officer with 'RDT doesn't test for impairment' consider the following;

  1. You have to be breaking the law to get drugs like Methylamphetamine in the first place.

  2. Police are 2 to 4 times more lenient than a laboratory in what they will consider 'positive'.

  3. Drug testing is intended to prevent drug users from using our roads. For those saying more people have died since RDT was invented; the road toll in NSW in 2006 (immediately before RDT became a thing) was 496 deaths (0.0073% of the population). In 2021 it was 270 deaths (0.0033% of the population). This is a huge drop and whilst you can't pin a drop in road fatalities on one safety initiative (vehicle design, speeding initiatives, driver education etc. have all contributed to a reduced road toll) the most idiotic thing to argue is that RDT has increased drug driving deaths.


It is illegal to drive whilst impaired by drugs/alcohol - even if you are not over the respective limits. But regardless of if you feel impaired or not, if you exceed the BrAC or have illicit drugs in your system - you will be committing an offence. Police do not write the laws they enforce. If you don't like the law as it is written, the only person who can change that is your local Member of parliament.

Fun drug testing trivia

  1. A roadside/field saliva test will not show you as positive for THC for 90 days despite what your stoner friend will tell you. Generally an oral fluid drug test only shows 12 - 24 hours of drug use and up to 2 days of chronic drug use.

  2. You can be positive for THC in urine for around 2 - 3 months if you are a chronic, heavy user of cannabis. This is why it is preferred to use saliva as a drug testing matrix in the workplace.

  3. You can have drugs in your system and come back negative. If you are under the cut-off levels set out in Australian standards, a laboratory has no choice but to report the finding and have you come back negative.

  4. NSW Police are randomly drug/alcohol tested. They have their hair pulled and this shows around 90 days of drug use. Quite comedically, in the case of Zisopoulos v Commissioner of Police [2018] NSWIRComm 1011 a Sergeant Zisopoulos tried arguing that he failed his drug test by swimming in Sydney Harbour which people may illegally dump drugs into.

  5. There are over 16,500 police officers in NSW, they have all heard the same conspiracy theories about RDT.

  6. This article is not legal advice, I am not your solicitor. If you need legal advice you should contact a solicitor registered with the Law Society of New South Wales.

358 views0 comments