Three Beers, an Antihistamine, and a Nine-Hour Shift
A 160 lb rider finishes a nine-hour shift at 11:00 pm, eats dinner, and drinks three 12-ounce regular beers between 11:15 pm and 1:15 am. At 10:00 pm he also took 25 mg of diphenhydramine for allergies. He feels fine, has no slurred speech, and lives four miles away on 35 mph city streets. Light rain has started and his visor fogs at stops. He is 24 years old, holds a motorcycle endorsement with no restrictions, and has never been arrested. His roommate offers to drive him home in a car instead, but the rider says he does not want to leave the bike out overnight.
1. Estimate the rider's blood alcohol concentration around 1:15 am and explain how much confidence should be placed in that number.
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A rough Widmark estimate:
- Body weight 160 lb ≈ 72.6 kg; distribution factor for a male ≈ 0.68.
- Three standard 12-oz regular beers ≈ 3 × 14 g = 42 g of pure alcohol.
- Peak BAC ≈ 42 / (72.6 × 0.68) ≈ 0.085%.
- Elimination ≈ 0.015% per hour × 2 hours ≈ 0.03%.
- Estimated BAC at 1:15 am ≈ 0.055%, with a plausible range of roughly 0.04–0.07% depending on drink strength, food, body composition and individual elimination rate.
That is above the 0.02% limit that applies to riders under 21 and below the 0.08% per se limit that applies to an adult. Confidence in the number should be low. Self-estimates are routinely wrong, elimination rates vary two-fold between people, and drinks poured at home are frequently stronger than 12 oz at 5%.
More important: at 24 he is subject to both the per se limit and the impaired-driving provision, which has no numeric threshold. A rider at 0.05% who weaves, reacts late or fumbles the controls can be charged and convicted on the officer's observations alone. 'Under the number' is not a defense.
2. What does diphenhydramine add, and how does it interact with the alcohol and the shift?
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Diphenhydramine is a first-generation antihistamine that crosses into the brain; drowsiness, slowed reaction time, reduced attention and blurred near vision are its known effects, not side effects.
Stack the three sources of impairment this rider has on board:
- Alcohol - reduces inhibition, slows processing, degrades balance and fine motor control. Balance is not a detail on a motorcycle; it is the vehicle.
- Sedating antihistamine - depresses the same systems. Alcohol and sedating antihistamines potentiate each other; the combination is more impairing than either alone at the same dose.
- Nine hours of work plus it now being past 1 am - the circadian low point, when alertness drops regardless of willpower.
It is legal to buy the medication. That does not make it legal or safe to ride with it, particularly mixed with alcohol at 1 am. If a rider is charged, the medication is part of the evidence, not an excuse. Two legal substances plus fatigue is a textbook impaired-riding profile.
3. What is the correct decision, and what does 'physical control' mean for the plan of leaving the bike overnight?
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Decision: do not ride. Take the car ride, retrieve the bike tomorrow afternoon.
Reasoning: the rider is impaired by alcohol, impaired by a sedating drug, sleep-deprived, and facing rain, fogging visor and 35 mph streets with cross traffic - four independent risk multipliers in a four-mile trip. Four miles is not a short exposure; most crashes happen close to home precisely because that is where most riding happens.
On physical control: impaired-riding statutes reach operating, not just moving. Sitting on the motorcycle with the engine running, walking it under power, or balancing it upright at the curb with the key in the ignition have all been treated as physical control. So the plan of 'I will just sit on it and warm it up' or 'I will push it home with the engine on' does not solve the problem - it creates one.
Practical steps: lock the bike, hand the keys to the roommate, and plan the retrieval for a time when the rider has slept, has eaten, and is not taking the antihistamine. Waiting two hours is not a plan; elimination takes hours and the drug and fatigue outlast the alcohol.