A prototype for a wearable signaling device for cyclists is presented. Based on Natural User Interface design concepts, the prototype uses an accelerometer to automatically detect motion changes from turning and braking and to trigger appropriate warning lights in the vest. The system is integrated into a vest made of hard, waterproof cardboard, folded for wearing comfort. Field tests demonstrated that the device detects body movements that anticipate a directional change, and outputs corresponding signals using LED lights without active control from the cyclist. To predict motion changes, a second sensing unit was installed to detect cyclist hand gestures commonly used to inform others of upcoming turns and stops.