In vehicles, especially road vehicles, devices are becoming increasingly well known which indicate the status of the road to the driver. Among these are ice warning indicators used together with on-board computers which trigger a warning indication within a temperature range in which icy road surfaces can be expected. However, these ice warning indicators have the disadvantage that although they do indicate the critical ambient temperature range, they provide no information as to the actual state of the road surface at that time or as to the road surface adhesion factor which is of importance in terms of the drivability of the road, because the adhesion factor does not automatically and directly vary with the ambient temperature.
The provision of devices indicating aquaplaning has also been proposed; such devices trigger a warning whenever a wheel, because of aquaplaning, loses its capacity to transmit friction to the road surface and thus loses peripheral velocity. Tests with various vehicles and various tires have demonstrated, however, that the state of aquaplaning, which is extraordinarily critical for vehicle controllability, occurs within a very narrow velocity range. The above-mentioned decrease in peripheral velocity occurs at a velocity which is only a few percent below a threshold velocity at which the wheels, due to aquaplaning conditions, are no longer capable of transmitting frictional forces. As a consequence, the aquaplaning warning comes, effectively, too late.
It is known to equip vehicles having power-assisted brakes with antiskid brake systems, that is, systems which by means of transducers ascertain variables during driving. Control variable signals are derived from the variables for varying the braking pressure. Such antiskid brake systems are described, for example, in "Bosch Technische Berichte" [Bosch Technical Reports], volume 7 (1980), No. 2 or in SAE Technical Paper No. 790458.