Cooperative collision warnings (CCWs) is one of the important applications of Vehicular Ad-Hoc Networks (VANETs) where secure and timely delivery of messages to the neighboring vehicles are needed. Secure communication is as important as timely communication to take proper actions in order to avoid collisions and thus prevent fatal accidents. However, security and delay are two competing metrics since security brings additional processing overhead, increasing the packet delays. While symmetric-key-based security techniques can be more efficient as opposed to public-key cryptography (PKC) in terms of delay, they introduce significant key maintenance overheads with the increased number of vehicles in VANETs. To alleviate this overhead and take the advantage of faster processing, we exploit the natural group behavior in CCW applications. We propose a delay efficient authentication scheme for VANETs which is based on group communication. Groups are created and maintained dynamically led by leader vehicles. Since the data communication within the groups will be dominating the overall packet traffic, we utilize symmetric-key techniques within each group which is handled by the group leader. Group creations on the other hand are less frequent events and thus are done by PKC. We analyzed the security properties of our proposed scheme and tested it with real-world vehicle data. Simulations results confirmed the efficiency in terms of delay with respect to other existing techniques. Copyright © 2011 John Wiley & Sons, Ltd.