A method and a device for outputting a signal when there is a hazardous surface under a vehicle includes providing an optical surface sensor for determining one or more parameters of the underlying surface, a stationary-state-determining device for determining whether the vehicle is stationary or is to be brought to a standstill, and an output device that outputs a signal if the underlying surface has been determined to be hazardous on the basis of the parameter(s), and the vehicle is stationary or is to be brought to a standstill.