We propose a pre-authentication method based on the scalable robust authentication protocol(SRAP). The SRAP features scalability and robustness by using group key signature system and R2V communications. Furthermore, it satisfies security issues such as confidentiality, privacy, and authentication, which are critical in VANETs environment, and outperforms previous researches. However, with SRAP, key request and authentication are repeatedly perform between a vehicle and an RSU, even if the vehicle is authenticated by the previous RSU. Our scheme modifies the key request stage of the SRAP and reduces the number of packets transmitted in the key reuqest stage which means that delays related to making and transmitting packets are reduced. In addition, calculation time is also decreased by using symmetric key encryption function. In this respect, our scheme makes efficient authentication process.