A method initiates a reaction of a first vehicle to a person in the environment of a second vehicle. The person is on a road on which the first vehicle is travelling and the second vehicle is detected. Measurement data determined by at least one sensor unit are received by a control unit. The control unit carries out a classification and registers the evaluated data as a person and as a second vehicle. Motion vectors of the person are determined and the expected movement of the person is calculated. A position and width of the vehicle doors of the second vehicle are determined or estimated on the basis of the measurement data. A probability of the person opening a vehicle door is calculated. A reaction of the first vehicle is initiated by the control unit depending on the calculated probability.