These questions test classification before measurement: age, licence condition, specimen type and substance. Identify those facts first, because a number that is right for one driver or specimen can be completely wrong for another.
Classify, then measure
Begin with age. A driver under 20 has a zero-alcohol limit, so food, apparent coordination or a small quantity does not create an allowance. For an ordinary driver aged 20 or over, first confirm that no special zero condition applies. If the person does not hold a zero-alcohol or alcohol-interlock licence, the ordinary ceiling is 250 micrograms of alcohol per litre of breath. For blood, the corresponding ceiling is 50 milligrams per 100 millilitres. Keep the units attached to the figure: breath and blood are different measurements and their numbers are not interchangeable.
| Driver or test | Rule to apply |
|---|---|
| Driver under 20 | Zero alcohol. |
| Ordinary driver aged 20 or over without a special zero limit | No more than 250 micrograms per litre of breath or 50 milligrams per 100 millilitres of blood. |
| Zero-alcohol or alcohol-interlock licence | Driving is an offence when the breath or blood contains any alcohol. |
| Drug-driving assessment | Legally supplied medicine and illicit drugs can both fall within the rules. |
For drugs, do not use legality of possession as a safety test. A prescription medicine can affect driving, and listed thresholds can apply to both prescription medicines and illicit drugs. The current initial oral-fluid screen is narrower than the later laboratory stage: it screens for THC, methamphetamine, MDMA and cocaine. A positive roadside sample can then move to broader laboratory testing. This sequence explains why a substance can matter under the wider drug-driving framework without appearing among the four names on the initial roadside screen.
The traps
- Halving the ordinary blood-alcohol ceiling. For the stated 20-plus driver without a special zero limit, the measured ceiling is 50 milligrams per 100 millilitres.
- Treating legal supply as exclusion from drug-driving rules. Prescription medicines and illicit drugs can both fall within the rules when the drug affects driving or reaches a listed threshold.
- Waiting for a crash before a drug matters. Drug-driving rules apply through the effect on driving and listed thresholds; a collision is not a prerequisite.
- Adding nicotine to the four-substance initial oral-fluid screen. The initial screen names THC, methamphetamine, MDMA and cocaine before broader laboratory testing.
Try it
An 18-year-old plans to drive home after a family celebration. What alcohol limit applies under the national rules?
- One standard drink if taken with food
- The same measured limit as drivers aged 20 or over
- No alcohol at all
- Any amount that does not visibly affect driving
A driver holds an alcohol-interlock licence. Which reading is enough on its own to make driving an offence, even if it is below the ordinary age-based limit?
- Only a reading above the court-offence threshold
- Only a reading that also impairs visible coordination
- Any reading above the limit for a full unrestricted licence
- Any detected alcohol