Intervehicle communication gives vehicles opportunities to exchange packets within limited transmission ranges and self-organize with ad hoc manner into VANETs (vehicular ad hoc networks). However, due to shared spectrum and present collisions' resolution mechanism which reject new admissions when wireless medium is busy, communications between vehicles have experienced severe throughput degradation and been restrained for concurrent transmissions. In this paper, a PCM (proximity-based concurrent transmission MAC) protocol has been proposed to permit concurrent access in shared channel for improving goodput in VANETs. By introducing game theory to the concurrent transmission opportunities determination process, PCM could provide extra access chances between active nodes or vehicles based on our defined proximity. To make PCM practical, we further give a detailed implementation method on NS2 to evaluate its performance. Numerical results show that PCM is not only feasible and reasonable in theory, but also has great improvement on average transmission delay, delivery ratio, and throughput performance in test.