Merging, e.g., from a ramp to a highway, is a potentially complex operation that frequently requires cooperation between the involved vehicles. It can also lead to significant disturbances of the traffic and even to accidents - either concerning the entering vehicles or induced ones due to the vehicles? reactions on the main lane. While V2X and automated driving might solve the problem over the long term, mixed traffic, including human-driven vehicles without connection, is expected to be the norm for a longer time. Against this background, this paper is concerned with a cautious merging assistant, which not only ensures the safety of the merging vehicle but also minimizes the disturbance of the main traffic. As an indicator of the induced traffic disturbance, we propose a function related only on readily available measurements of the first disturbed vehicle. The merging assistant is designed as an MPC control using a novel reaction model. The evaluation is done using a catalogue of standard merging situations and shows that the cautious control does indeed provide a safe merging while affecting less the incoming traffic.