V2V Safety Communications in the US and Europe will utilize the IEEE 802.11p MAC protocol, which provides a frame prioritization mechanism known as Enhanced Distributed Channel Access (EDCA). Safety messages can often be classified by the sender as "routine" or "critical," and placed in different Access Categories (ACs) to give the critical messages priority channel access. In this paper we address the problem of how to further classify otherwise "routine" safety messages into priority classes. In particular, we describe an algorithm that computes a metric called Suspected Tracking Error (STE) and assigns the priority class of a message based on the current value of STE. Packets with the largest STE are assigned to the highest priority class, along with those classified as "critical" according to other criteria. Packets with moderate STE are assigned to the lower priority class. While other approaches using STE have emphasized giving more transmission opportunities to vehicles experiencing high dynamics, this approach instead provides the advantage of higher probability of successful delivery. Simulation results demonstrating the benefit of the proposed algorithm are provided.
