PURPOSES: The sensitivity of $CO_2$ emissions per vehicle by a various speeds is compared according to the type of roads. METHODS: The methodology of the study are as follows: First, the sensitivity of $CO_2$ emissions per vehicle are analyzed by averaged daily travel speeds. Second, the sensitivity of $CO_2$ emissions per vehicle are analyzed by averaged hourly travel speed. Third, the sensitivity of $CO_2$ emissions per vehicle are analyzed by sectional travel speeds. RESULTS: The sensitivity that on Saturday in a week, at peak times in a day and in close location from Seoul was higher than in other situations. CONCLUSIONS: From this study, we may conclude that $CO_2$ emissions per vehicle at low speeds are generally more sensitive.