Communication adapter foreign to a vehicle, which is portable and which can be coupled reversibly by means of a communication interface with a control apparatus of a motor vehicle, wherein the communication adapter is designed to operate a communication connection, by means of an air interface on the basis of radio waves, infrared waves or microwaves, to at least one further, in particular autonomous, motor vehicle and/or a coordination device external to the vehicle, wherein the communication adapter is furthermore configured to receive from the control apparatus motor vehicle data describing a traffic behaviour and/or a traffic environment of the motor vehicle, wherein the communication adapter has a transceiver unit and is designed, by means of the transceiver unit, on the one hand to transmit the motor vehicle data to the coordination device external to the vehicle and on the other hand to receive the warning data generated via the air-interface-based communication connection by the coordination device external to the vehicle and describing a traffic behaviour and/or a traffic environment of at least one other motor vehicle and to trigger on the basis of the warning data at least one traffic-coordinating measure, wherein the traffic-coordinating measure consists in an outputting of a warning to a driver of a motor vehicle, characterized in that the communication adapter is designed to write the motor vehicle data first in a predetermined cache and to transmit them by means of the transceiver unit as a function of the presence of a fulfilled release criterion to the coordination device external to the vehicle, wherein if the release criterion is not fulfilled, no transmission takes place, and wherein the release criterion consists in a successful authentication and/or a successful proof of identity of another motor vehicle requesting the motor vehicle data. wherein the communication adapter is designed to operate a communication connection, by means of an air interface on the basis of radio waves, infrared waves or microwaves, to at least one further, in particular autonomous, motor vehicle and/or a coordination device external to the vehicle, wherein the communication adapter is furthermore configured to receive from the control apparatus motor vehicle data describing a traffic behaviour and/or a traffic environment of the motor vehicle, wherein the communication adapter has a transceiver unit and is designed, by means of the transceiver unit, on the one hand to transmit the motor vehicle data to the coordination device external to the vehicle and on the other hand to receive the warning data generated via the air-interface-based communication connection by the coordination device external to the vehicle and describing a traffic behaviour and/or a traffic environment of at least one other motor vehicle and to trigger on the basis of the warning data at least one traffic-coordinating measure, wherein the traffic-coordinating measure consists in an outputting of a warning to a driver of a motor vehicle, characterized in that the communication adapter is designed to write the motor vehicle data first in a predetermined cache and to transmit them by means of the transceiver unit as a function of the presence of a fulfilled release criterion to the coordination device external to the vehicle, wherein if the release criterion is not fulfilled, no transmission takes place, and wherein the release criterion consists in a successful authentication and/or a successful proof of identity of another motor vehicle requesting the motor vehicle data.