Police roadside drug screening tests in Canberra went down last year but the "hit rate" of positive readings went up, and the number of drug-affected drivers was just 11 per cent less than the drink-drivers.
In 2018-19, police took 3541 oral driver fluid swabs in the ACT to assess the presence of cannabis, methamphetamine or MDMA (ecstasy), 228 fewer than in 2017-18.
However, police success in identifying and apprehending drug-affected drivers in Canberra almost doubled, with 1004 drivers recording positive in 2018-19, as opposed to 556 in the previous corresponding year.
Roadside screening tests for alcohol, which are much simpler and quicker for police to perform, increased to 97,885 across the ACT in 2018-19, a 10.9 per cent increase over the previous corresponding period.
However, the police "hit rate" on roadside breath testing is much lower than the drug screening, with 1121 drink drivers apprehended in 2018-19 with a blood alcohol reading higher than 0.05, up from 1097 in 2017-18.
The discussion about drug driving and the costly screening tests involved in detecting affected drivers has become embroiled within the latest decision to legalise the cultivation and personal use of small quantities of cannabis in the ACT from January 31 next year.
The two-step roadside drug detection system used by police is procedurally similar to that of the alcohol test but with a significant difference: the drug test has no inbuilt tolerance level.
If a driver tests positive to the presence of one of the drugs, and this is confirmed by the secondary test, then the charge is laid.
In addressing the Standing Committee on Health, Ageing and Community Services in May, ACT Chief Minister Andrew Barr said he "did not envisage that changes [to cannabis laws] would change drug-driving laws".
"I note that there is a difference, as I understand it, between how long cannabis stays in your system, in the detectable sense by way of a drug test, and how long alcohol does," he added.
Several times this year the government repeatedly has flagged no intention of changing its zero-tolerance stance on drug-impaired driving.
Road Safety Minister Shane Rattenbury, in response to a question from the ACT Liberals in February this year, described drug impairment levels as "an uncertain area of science".
"There is continuing scientific research on how long these drugs remain detectable in your system and for how long they can cause impairment," Mr Rattenbury told the Assembly.
"But I want to be very clear that if cannabis is decriminalised in the ACT, clearly we will need to be very deliberate in educating the community about the risks of consuming cannabis and driving."
ACT police will not disclose the cost of a roadside drug test due to "commercial in confidence" provisions however, NSW police were openly prepared to do so three years ago.
The costs between jurisdictions are comparable, with NSW obtaining a more favourable rate than the ACT for its test kits because it is a larger volume customer.
In 2016, the cost of an initial roadside drug test in NSW was $25.50 per test. This is simply the cost of the device only and does not include police wages, vehicle usage costs and consumables such as protective gloves.
If the first oral swab from the driver returns a positive test, then the driver is asked to participate in a second screening test which costs more than $31 in NSW.
In the US, where many of the states have a longer experience with decriminalisation of cannabis and how this may affect road safety, the complete picture still isn't clear because not all drug-impaired driving data is collated the same way.
In assessing data from 2016, the non-for-profit Governors Highway Safety Association, which conducts research across the US to assess road safety trends and issues, reported that 44 per cent of fatally-injured drivers with known results tested positive for drugs, up from 28 per cent 10 years previously.