The shift towards autonomous driving will allow people to read and work while riding in such a vehicle. Some people suffer from motion sickness while being engaged in these activities in a moving vehicle since the signals of the visual and vestibular systems of the human body are mismatched. One approach to mitigate motion sickness is to provide visual cues for the affected person. We present Bubble Margin, a smartphone application to reduce motion sickness while reading in a moving vehicle. It is built as a stand-alone, semi-transparent overlay application that visualizes motion cues in form of bubbles at the margins of a smartphone while other applications may be used in the remaining area. Forces to move these bubbles are taken from smartphone sensors. An initial within-subject design study (N = 10) with participants performing a reading task in a real driving scenario showed mitigating effects on motion sickness symptoms.