Criteria for stability and controllability of road vehicles are briefly reviewed, and it is argued that there is a need for criteria that might better relate to subjective ratings by drivers. The variance of a driver's closed-loop control action against random disturbances acting on the vehicle is proposed as a realistic criterion that might relate to a driver's assessment of the vehicle. A nonlinear vehicle model with five degrees-of-freedom, negotiating a 90-deg bend in minimum time, is the basis for the theoretical study. The vehicle model is run with the center of mass in two different positions. It is found that the variance of the driver's compensatory steering control varies significantly through the maneuver, reaching a peak at about midcorner. The corresponding variance in the lateral path error of the vehicle also peaks at about the same position in the maneuver. Comparison of these variances to existing stability and controllability criteria shows that the variance of the compensatory control might reveal aspects of the handling behavior that the existing criteria do not. Recommendations for further work are given and include a program of driving simulator experiments or track tests to correlate the new criteria against subjective ratings by human drivers.

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