This paper will evaluate the string stability property of a longitudinal control strategy designed to be safe, comfortable and robust. The human driving model is assumed to follow the pipes model. Results show that the control model is structurally string stable so that no servoloop controller is needed. As human driving model is string unstable, the authors show that only 5% penetration rate of the ACC model should guarantee traffic stability for some scenario in mixed environment. Such satisfactory result relies on a string stability margin (SSM) that is much higher than the SSM of some ACC control strategy for the given scenario. Because the control strategy is nonlinear, absolute stability and SSM value cannot be given. Yet the strategy seems sable for different scenarios.