We propose the guidance information protocol to provide the real-time decision support system to drivers or pedestrians using personal ITS station. The reference architecture provides a general structure for the real-time decision support system and the method of message exchange between the personal ITS station and the roadside ITS station. The proposed method is a flexible application protocol for safety warning and parking guidance services. This protocol makes the client part independent of use cases for supporting light-weighted devices. We aim to design the protocol to be flexible for accommodating various use-cases as well as suitable for being implemented on light-weighted personal devices.