In normal vehicle operation, drivers occasionally encounter low friction patches without advance warning. The sudden transitions to and from the low friction surface may destabilize the vehicle, even when the operating point on the higher friction surface is away from the handling limits. Because of these fast dynamics, an active assistance system could significantly enhance safety. This paper describes the dynamics of transition between friction surfaces and demonstrates the utility of an envelope controller that uses real-time friction estimates and a state-bounding approach. Experimental data and the controller state bounds are used to provide insight into the trade-off between estimator accuracy and response time when used for real-time control.