In the vehicular environment, the transmissions of signals have to face the higher challenges then indoor wireless local area networks (WLAN). Multipath fading cause high bits error rate (BER), in such an unstable transmission environment. And high BER cause high dropping probability of packets. In order to reduce the dropping probability of handshaking packets without increase the capability of infrastructures, a mechanism is proposed in this paper. In our work, we altered the traditional cyclic prefix (CP) into a variable cyclic prefix (VCP) of request to send (RTS) frame and clear to send (CTS) frame, and reduce the handshaking packets error rate in vehicular wireless transmissions, hence we can improve the contending ability in Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) at vehicles to infrastructure (V2I) transmission. , By analyzing environment parameters, we can predict the suitable VCP time slots and dynamic adjust VCPs. And we can competently eliminates the impacts of Intersymbol interference (ISI) caused by multipath fading in CSMA/CA.