Three beers and a probationary A2 licence
Mikkel is 20 and has held an A2 licence for 14 months. On a Friday he rides his 35 kW motorcycle to a party, arriving at 20:00. Between 20:00 and 23:00 he drinks three bottles of beer, each 0.33 L at 4.6% alcohol, and finishes the last one at 23:00. He eats dinner at 21:00 and a sandwich at 22:30. At 01:15 he feels sober, the roads are quiet, and his motorcycle is parked 11 km from home on a lit suburban route. He weighs 78 kg. He considers riding home because he has an early shift at 07:00 and a taxi would cost DKK 350. The weather is dry, he has no pillion, he takes no medication and has used no drugs. His only alcohol is the three beers. He describes himself as a confident rider and says the short distance makes it low risk.
1. Which blood alcohol limit applies to Mikkel, and why does it differ from the general limit?
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Mikkel has held his licence for only 14 months. In Denmark a stricter limit applies during the first three years after passing the test: 0.2 g/L (0.02%) rather than the general 0.5 g/L (0.05%). The reason is that novice riders are over-represented in single-vehicle crashes, and even small amounts of alcohol degrade balance, hazard detection and throttle/brake coordination before the rider feels drunk. The lower limit is not a guideline; exceeding it can lead to a fine, suspension and a conditional licence. The practical consequence for Mikkel is that he cannot use the general 0.5 g/L limit as a target. Any alcohol at all creates legal risk because the margin between 0.0 and 0.2 g/L is very narrow and individual elimination rates vary. He should treat the probationary limit as a reason to plan not to ride after drinking, not as a calculation exercise.
2. Estimate Mikkel's blood alcohol concentration at 01:15 using the information in the setup. State your assumptions and say whether he would be over the applicable limit.
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Alcohol mass: 3 × 0.33 L × 4.6% = 0.0455 L of pure alcohol, which is about 45.5 g. A Widmark calculation gives BAC = alcohol mass / (body weight × r). For a man, r is commonly taken as 0.68-0.70. With r = 0.70: 45.5 / (78 × 0.70) = 0.83 g/L at peak. The last drink was at 23:00, so at 01:15 about 2.25 hours have passed. Elimination is roughly 0.10-0.15 g/L per hour, giving a reduction of 0.23-0.34 g/L. Estimated BAC at 01:15 is therefore about 0.49-0.60 g/L. Even using favourable assumptions, such as r = 0.80 and elimination of 0.15 g/L per hour, the result is about 0.39 g/L. That is still well above 0.2 g/L. The estimate is not a precise measurement: absorption, food and individual metabolism vary, but the uncertainty does not change the conclusion. He would be over the limit, and he may still be impaired.
3. What should Mikkel do, and what would you say to counter his argument that an 11 km ride on quiet roads is low risk?
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He should not ride. He should lock the motorcycle, take a taxi, call someone sober, or stay overnight and collect the bike the next day. The 11 km argument is misleading because most alcohol-related motorcycle crashes happen on short, familiar trips where the rider relaxes. Quiet roads reduce traffic conflict but do not restore balance, vision or reaction time. At 50 km/h an 11 km ride takes about 13 minutes and covers roughly 180 m every 13 seconds; a delayed reaction or a wobble at a junction is enough. If he is stopped, the probationary limit applies and the consequences include a fine, loss of licence and conditions for regaining it. His insurance position can also be affected after a crash. A useful script is: 'I have had three beers, my limit is 0.2, and I am not riding. I will collect the bike tomorrow.' The early shift is a reason to plan sleep, not a reason to ride impaired.