The effectiveness of any braking system is ultimately limited by the point at which the wheels lock and slide over the surface on which the stop is being made. When surfaces are clean and dry the adhesion with a rubber tyre is high enough to meet most requirements of braking without risk of skidding. In wet, or in icy conditions, however, adhesion may be reduced to such an extent that, even with efficient brakes, stopping distances are greatly increased and dangerous conditions can arise. Under wet conditions there are special difficulties. Different surfaces may have very different skid-resisting properties and, in addition, when surfaces are wet, the adhesion with a tyre depends on speed, generally diminishing as speed is raised.