In Japan, vehicle horns have been used as a means of communication between drivers, including for the expression of gratitude. According to our previous survey, however, psychologically negative reactions are aroused not only in drivers but also in pedestrians by vehicle horn usage. The improvement of the sound quality of horns should be effective in reducing negative mental reactions among those who hear the sounds. In this study, psychoacoustic experiments were conducted to examine the effects of the acoustic properties of vehicle horns on auditory impressions, and, on the assumption that horns are heard both inside and outside vehicles, on estimations of the reason for another driver's horn usage. Results showed horns outside of vehicles were evaluated as more unpleasant than those heard inside because the former stimuli were louder than the latter. Further, impressions of unpleasantness were stronger as duration of horn use became longer. As for the judgment of another driver's reason for horn usage, short horns were often perceived to be used for driving etiquette such as gratitude and greetings, while horns used longer than 1 second were frequently perceived as venting anger. From these results, we concluded that impressions of pleasantness or unpleasantness induced by vehicle horns were related to the acoustic properties of horns. Furthermore, unpleasant perceptions of drivers may be partly caused by hearer interpretations of the driver's reason for horn usage. Significant relationships were found between impressions of unpleasantness and participant interpretations of horn use as representing the venting of anger.