In urban vehicular networks, where privacy, especially the location privacy of anonymous vehicles is highly concerned, anonymous verification of vehicles is indispensable. Consequently, an attacker who succeeds in forging multiple hostile identifies can easily launch a Sybil attack, gaining a disproportionately large influence. In Vehicular Ad Hoc Networks (VANETs), the vehicular scenario requires smart signaling, smart road maintenance and other services. A brand new security issue is that the semi-trusted Road Side Units (RSUs) may be compromised. The objective of our work is to propose a Threshold ElGamal system based key management scheme for safeguarding VANET from the compromised RSUs and their collusion with the malicious vehicles. By analyzing the packet loss tolerance for security performance demonstration, followed by a discussion on the threshold our method can promote security with low overhead in Emergency Braking Notification (EBN) and does not increase overhead in and Decentralized Floating Car Data (DFCD) during security promotion.