The effectiveness of DSRC for collision avoidance depends on the communication performance of safety messages. EDCA, the standard IEEE 802.11 QoS capability, was designed for networks with a mix of voice, video and best effort traffic. This paper examines how to use EDCA to reduce frame collisions for a channel dominated by periodic safety messages. The paper makes two main contributions: Access Category (AC) Isolation and Virtual Division. AC Isolation eliminates inter-AC countdown collisions, dramatically improving the success rate for high priority packets. Virtual Division uses isolation in a novel way that also reduces collisions for lower priority packets. Both techniques are consistent with the 802.11 standard. The paper includes both detailed analysis and insightful NS-2 simulations.