Operating a Smartphone to use a mobile service while driving can be dangerous by distracting users from driving. Recognizing user's context of driving and automatically activating a corresponding mobile service can be a possible way to reduce user's manual operations and provide a better driving experience with Smartphone in a safe way while driving. In this paper we present an efficient method for recognizing user contexts of driving based on accelerometer signals of Smartphone. Our method uses well-designed features that are not only effective to discriminate driving contexts but also efficient to compute on a Smartphone device in real-time. Throughout real-world data set, we also demonstrate some of useful behaviors of our proposed method.