A communication system includes a motor vehicle, a portable communication adapter separate from the motor vehicle and able to be coupled reversibly to a control device of the motor vehicle via a communication interface, and a vehicle-external coordination device. The communication adapter receives motor vehicle data describing a traffic behavior and/or a traffic environment of the motor vehicle from the control device. The communication adapter includes a transmission/reception unit and transmits the motor vehicle data to the vehicle-external coordination device via the transmission/reception unit and receives warning data generated by the vehicle-external coordination device which describes a traffic behavior and/or a traffic environment of another motor vehicle. The communication adapter initiates a traffic-coordinating measure based on the warning data.