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A haptic interface, also called haptic display, is a system that informs and aids a human operator by forces on the control device (stick, steering wheel or other). These interfaces are being explored for many fields, e.g., for UAV control, (tele-)robotics, automotive control and flying. The force feedback helps in control tasks and increases the operator’s awareness. Proper design of such interfaces promotes “shared control”, where an autonomous agent and the human operator can jointly exercise control on a dynamic system. The human’s flexibility and adaptivity of his neuromuscular system offers ways to override the haptic support, should this be necessary. Haptic interfaces require design decisions on three issues: (a) The appropriate guidance laws should be developed, thus the behavior of the automated agent must be defined. This guidance should be inherently safe and useful, and it should be compatible with human control strategies, (b) The guidance should be translated to haptic input on the control device. Here additional force and modification of the control device’s apparent properties (mass, damping, spring coefficients) can be used, and (c) The scaling between the guidance and the haptic input should be tuned to the proper level. From the above, it appears possible to break down the design process into individual steps. However, in a recent research project in which individualized guidance laws were investigated, we discovered an interaction between the guidance laws and the perceived haptic feedback strength, where variation in the guidance laws produced an apparent change in haptic authority by the automation. This paper discusses this experiment – car driving with lateral support – and analyses the causes of the interaction. The results include recommendations for removing this interaction.