A safety belt warning system for vehicles comprises a sensor mat for the detection of a force acting on its surface, which includes, for the formation of two electrical capacitors, two dielectric layers which are located one above the other in sandwich fashion and arranged in each case between electrically conductive coats, and which have different compressibility at least in the direction of loading due to force, so that the capacitances of the two capacitors vary differently with a load on the sensor mat.