A driver has had two drinks and also used cannabis earlier in the evening, believing the substances offset each other — one stimulating alertness, one calming. What does research show about combining alcohol and cannabis before driving?

Why A is correct

Alcohol and cannabis impair through different mechanisms, and 'offsetting' is a folk theory with no support in the driving data: the combination impairs reaction time, lane-keeping and divided attention at least additively and by some measures synergistically. Worse, the perceived impairment is smaller than the measured impairment, because each substance's subjective signature masks the other's — the classic setup of a driver who feels functional and measures terrible. Crash-risk studies find the combination multiplies rather than adds, which is the empirical answer to the folk theory.

Why the other options are wrong

B. The combination's effects are identical to the effects of the alcohol alone

The effects are not merely additive; studies of combined use find impairment greater than the sum of the parts on core driving measures. Treating the combination as 'just the alcohol' undercounts both the measurable deficit and the crash risk.

C. The combination is lawful as long as each substance individually was used legally

Legality of each substance at home says nothing about the driving standard, which is impairment by any drug. Two lawful substances do not average into a lawful state behind the wheel any more than two legal left turns make a legal U-turn.

D. They counteract each other, so the combination is safer than either alone

Stimulant versus depressant is the wrong frame: neither substance is safely 'cancelled' by the other, and the skills that driving needs — tracking, timing, divided attention — are degraded by both, in different ways, at the same time.

On the road

The house party's standard line — 'the beer and the green cancel out' — meets its rebuttal in the state patrol's annual data: the two-substance crashes are the worst ones. The driver who mixed both and felt fine drifts across a centre line that a driver of either substance alone might have held. The folk theory has a body count.

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