This paper discusses multi-dimensional aspects of "kashin" on a forward vehicle collision avoidance system. The term "kashin" could refer to driver over-trust, over-reliance, complacency (lack of vigilance), or risk compensation. However, those notions may be mutually related to each other in a very complicated manner. We try to clarify the relationship among the "kashin"-related notions in order to point out what should be done for designers of forward vehicle collision avoidance systems to prevent drivers' "kashin."