Vehicular Ad Hoc Networks (VANETs) provide communication between vehicles and roadside infrastructure for safety, traffic management, and intelligent transportation applications. However, the highly dynamic nature of VANETs exposes vehicular communication to security threats including unauthorized access, spoofing, Sybil attacks, data tampering, and privacy violations. This study presents a blockchain-based security model for enhancing data integrity, privacy, authentication, and communication reliability in VANETs. The proposed framework integrates a blockchain layer with a Modified Stable Weighted Clustering (m-SWC) algorithm, cryptographic authentication, pseudonymous identities, and a consensus mechanism. The model was evaluated through simulation under different vehicle densities and attack conditions. Performance was assessed using packet delivery ratio, end-to-end delay, authentication success rate, attack rejection, communication overhead, and scalability. The results show that the proposed blockchain-enabled VANET achieves higher and more stable packet delivery performance than conventional approaches. Authentication performance also improved, with the Blockchain VANET achieving a 97% authentication success rate and 95% attack rejection compared with 86% and 78%, respectively, for the PKI system. The results further show that blockchain introduces moderate additional latency because of consensus and verification processes, but the delay remains within acceptable limits for real-time vehicular applications. The system also demonstrates controlled communication overhead and robust scalability as the number of vehicles increases from 50 to 200. The study therefore demonstrates that blockchain integration provides an effective balance between security, reliability, and scalability in VANET environments.