—In the context of vehicular safety and entertainment applications, we focus on the design of a reliable medium access control scheme. Each vehicle is willing to form a network and regularly communicate with the other vehicles in its vicinity, forming a Vehicular Ad hoc Network (VANET). The information that is being communicated is short lived, and requires a very low probability of failure. In this article we assume that there is no central entity managing the medium access for the cluster of vehicles. Several code-based repetition-based QoS provisioning MAC schemes have been proposed for this scenario. We present a mathematical description of the frame failure probability, our performance metric, and explain its application in finding optimal codes.