There exist various communication methods among drivers on roads by using functions of vehicles and drivers' body such as high beam and hand waving. These “ Car Body Languages” have emerged in drivers' society spontaneously in order to overcome impossibility of communication among drivers. This study focuses on one of such CBDs, “ high-beam”. High-beam is used commonly as two opposite meanings in Japan:‘ warning’ and ‘ giving the right of way’. The risk of such a duality in meaning of CBD and its social stability in drivers' society are analyzed by employing a simulation opf the evolutionary model of conventions where drivers repeat learning on other drivers' recognition of the meaning of ‘ high-beam’.