A characterization of dynamic human braking behavior with implications for ACC design

Skilled driving behavior can be characterized as tracking, control, and regulation of appropriate perceptual cues. Because of environmental complexity, drivers must restrict attention to appropriate perceptual cues and act to cause their vehicle to be in an acceptable perceptual state space. From experiments and supporting literature, we identify time headway and time-to-collision as plausible perceptual cues, and characterize skilled braking behavior as a trajectory through the resulting perceptual state space. This trajectory, which terminates at a desired time headway value and infinite time to collision value, evolves in a smooth counter clockwise direction in the perceptual space spanned by time headway and inverse time-to-collision. Experimental evidence suggests that if automated braking, such as those required in emerging adaptive cruise control (ACC) systems violates the smooth counterclockwise characteristics of this human-generated perceptual trajectory then human subjects perceive the automated braking as unnatural or uncomfortable. Consequently, to produce comfortable performance ACC designers need to develop controllers that emulate this desired perceptual trajectory.