With significant advances in the development of autonomous cars and Internet of Things (IoT) in recent years, Vehicular Ad-hoc NETworks (VANETs) have become a promising technology for Intelligent Transportation Systems (ITS) as well. However, the wireless nature of VANET communication makes it vulnerable to a plethora of attack vectors to otherwise secure vehicles. Authenticated message dissemination plays a key role to avert such security vulnerabilities. Many of the existing Public Key Infrastructure (PKI) based schemes use Certificates for authentication. In such schemes, for authenticating an entity which presents its certificate, Certificate Revocation List (CRL) is used to check if the entity's certificate has been revoked. But, as the size of the CRL grows, using CRL for authentication can incur computation and storage overhead in VANETs. To overcome this limitation of CRL-based approach for authentication, in this paper, we propose a distributed, scalable, low-overhead, privacy-preserving authentication scheme for VANETs. The proposed scheme uses a Merkle Hash Tree (MHT) for authenticating Road Side Units (RSUs) and Modified Merkle Patricia Trie (MMPT) for authenticating vehicles. We also present an informal analysis as well as formal correctness proof of the proposed scheme.