A device for determining a driving state of a two-axle motor vehicle, which driving state is formed from the driving situation of the motor vehicle and the state of the underlying surface, wherein output signals from vehicle on-board sensors are evaluated in order to determine the driving situation and wherein a routine for determining and/or estimating the friction value of the underlying surface is provided in order to determine the state of the underlying surface. Also disclosed is a method for operating a combined vehicle brake system, in particular for motor vehicles, having hydraulically actuable wheel brakes at a front axle and having electromechanically actuable wheel brakes at a rear axle, wherein the vehicle wheels assigned to the rear axle can be driven at least at times by an electric motor which can be operated as a generator for the recuperation of braking energy