Impairment questions mix three different decisions: whether ability is compromised, which numerical alcohol boundary applies, and whether age or novice status creates a zero condition. Classify the driver and the rule before comparing a reading or a detected substance.
Keep impairment, alcohol levels and zero conditions separate
Start with actual impairment. Alcohol, cannabis, prescription medicine, non-prescription medicine, illegal drugs or a combination can all fall within the rule when they compromise driving ability to any degree. The source of a substance does not decide the issue by itself. A legally supplied medicine that has no effect on driving ability does not meet that impairment premise, while a legally supplied medicine that does reduce ability can.
| Boundary | How to apply it |
|---|---|
| BAC 0.05–0.079 | This is the provincial warn range, not a safe-driving allowance. |
| BAC 0.08 or more | This is the separate level at which criminal charges can arise. |
| Zero alcohol | Apply it to anyone aged 21 or under and to every G1 or G2 holder. |
| Zero drug after approved screening | Apply the novice branch at any age and the full-G age branch only below 22. |
For alcohol, do not merge the provincial warn range with the criminal-charge threshold. A reading from 0.05 through 0.079 is already within the warn range, while 0.08 or more is the distinct criminal boundary. Neither figure creates a safe target. The zero-alcohol category is stricter: it covers every driver aged 21 or under and every G1 or G2 holder, including an older novice. A full G holder moves outside the age branch at 22, but the general impairment rule continues to apply.
For the approved-screening zero-drug condition, use two branches. G1 and G2 status is enough regardless of age; for a full G holder, the young-driver branch applies only below 22. Medical authorization is not automatic clearance. An officer must be satisfied that the use is lawfully authorized and that the substance detected is the one authorized. Even when those predicates are met, driving while ability is compromised remains illegal.
The traps
- Treating every prescribed medicine as impairment. The general impairment rule turns on compromised driving ability, not prescription status alone.
- Treating the warn range as permission or a safe target. A reading from 0.05 through 0.079 is an alcohol-control boundary, not clearance to drive.
- Assuming a full G licence cancels the age-based zero-alcohol condition. Drivers aged 21 or under remain in the zero category.
- Extending the young-driver zero-drug category past its boundary. For a full G driver, that age branch covers people under 22; novice G1 and G2 status is a separate branch at any age.
Try it
At what blood-alcohol concentration can an Ontario driver face criminal charges? Enter the BAC as a decimal value.
Approved screening equipment detects cannabis in a G2 driver's body. The driver has a medical authorization for cannabis but is visibly impaired. Which statement is accurate?
- The authorization automatically cancels both the zero-drug condition and impaired-driving law
- The exception applies whenever any prescribed drug is detected
- Police must be satisfied about legal authorization and the matching drug; impairment remains illegal
- The exception applies only if the driver has no licence