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Vehicular ad hoc networks (VANET) is the foundation of intelligent traffic system (ITS), and many MAC protocols for VANET are proposed, among which a reliable MAC protocol called ad hoc has aroused much attention. By investigating the details of ad hoc protocol, we have discovered several unsolved problems that might lead to a network failure. In this paper, we provide a quantitative analysis of the success probability for contending nodes and prove that an adaptive frame length is quite necessary. We propose adaptive-ADHOC (A-ad hoc) MAC protocol, which implements a robust mechanism supporting the adaptive frame length. Evaluation result shows that A-ad hoc can maintain a high contending success probability and obtain about 50% reduction of response time over original ad hoc protocol, while providing important enhancement on network robustness.