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Vehicle Safety Communications (VSC) using Roadside-to-Vehicle (R2V) and Vehicle-to-Vehicle (V2V) communications have attracted much attention for recent years. In Japan, a single 10MHz bandwidth channel on 700MHz band is allocated for VSC. Therefore, R2V and V2V communications have to share the same channel. Additionally, R2V communications require higher quality than V2V communications. Therefore, interference from V2V to R2V communications caused by the hidden terminal problem is a critical issue. This paper proposes Prioritized-CSMA (PR-CSMA) protocol which can reduce interference from V2V to R2V communications by dividing time into R2V slots and V2V slots. To achieve this, RSU distributes R2V slot information (SI) to OBEs within a communication range. Furthermore, to deliver the SI to hidden terminals, OBEs that received the SI forward it to other OBEs. We evaluate two different SI distribution schemes and show that the proposed protocol has capability to reduce interference from V2V communications.