In this situation, are you allowed to use the horn?

In this situation, using the horn is not allowed. According to Art. 29 para. 2 of the Road Traffic Act, the use of an audible signal in a built-up area is prohibited unless it is necessary due to immediate danger. Furthermore, seeing a pedestrian with visibly limited mobility (cane), you are obliged to stop the vehicle and allow them to cross, as required by Art. 26 para. 7.