Should you use the horn in the presented situation, in order to warn the cyclist of your intention to overtake?

In the video, you are driving a lorry along a street in town, and in front of you, at the right-hand edge of the roadway, there is a cyclist whom you intend to overtake; nothing indicates immediate danger. You are not allowed to use the horn to warn the cyclist that you are overtaking. The answer is: no.

The Road Traffic Act states that a driver may use a sound or light signal when it is necessary to warn of danger, and it prohibits using the horn in a built-up area unless it is necessary because of immediate danger. Normal overtaking is not such a danger - it is a routine manoeuvre which you perform keeping a lateral clearance of at least 1 m from the cyclist; the cyclist is indeed required to move as far to the right as possible to make overtaking easier, but you must not force them to do so with the horn. A lorry horn right behind their back may frighten them and make them wobble or swerve under your wheels - the opposite of what you intended. You use the horn only when, for example, the cyclist starts turning left without seeing you, or when a car is coming out of a gateway - then the signal is necessary.

Legal basis: Art. 29 para. 1 and para. 2 point 2 and Art. 24 para. 2 and 6 of the Road Traffic Act (Prawo o ruchu drogowym)