1. Alcohol differently affects each individual

Breath alcohol content calculations on the Internet or via an app usually take into account your age, gender, size, and weight. Further individual factors such as illness, medication, how full your stomach is, or the individual breakdown rate will affect the impact.

2. Hardly anyone keeps a precise record of what they have drunk

In the pub, you're given the bill at the end of the evening - making it possible to keep track of your alcohol consumption in order to estimate your breath alcohol content. At a private party, however, it is not so easy to remember what you have drunk and how much, and that becomes more difficult with each glass. In addition to the total amount of alcohol consumed, you need to take into account the temporal distribution of consumption over the night (or day).

3. At what point is an individual unsafe to drive?

Alcohol can have an effect at a breath alcohol content of only 0.3 - the equivalent to the amount of sparkling wine that Claudia had. On the one hand, the person is relaxed and in a cheerful mood. But on the other hand, her driving ability could be affected by a slight reduction in visual performance, decreased concentration, reduced power of judgement and delayed reaction times, as well as an increased appetite for risk-taking. If this results in poor driving or leads to an accident, this 'relative unfitness to drive' is deemed as a criminal offence. The likelihood of being involved in an accident when in a slightly drunken condition with a breath alcohol reading of 0.5 is already twice as high as with 0.0*.

_{*Source: TÜV Süd, http://www.tuev-sued.de/fuehrerschein_pruefung/aktuell_informiert/alkohol_und_drogen_im_strassenverkehr/wirkung_von_alkohol (Retrieved: 12.11.2015)}