In this paper, we introduce a novel Public Key Infrastructure (PKI) based message authentication scheme that takes advantage of temporal and spatial uniqueness in physical layer channel responses for each transmission pair in vehicular communication networks. The proposed scheme aims at achieving fast authentication and minimizing the packet transmission overhead without compromising the security requirements, in which most messages can be authenticated through an extreme fast physical-layer authentication mechanism. We will demonstrate that the proposed secure authentication scheme can achieve very short message delay and reduced communication overhead through extensive analysis and simulation.