In vehicular networks (VN), the response time is critical, whereas, an autonomous and efficient way of preventing hazardous situations on roads plays an important role in the successful deployment of the system. Trust or reputation models often need to be integrated with inter-vehicle communication protocols in use to avoid selfish or malicious behavior by the vehicles exploiting the system. Existing trust and reputation models for VNs lack the capability of fast and accurate trust management suitable for the ephemeral association of vehicles. In this paper, we design an augmented trust model for VNs by assigning each vehicle a long term trust value. Our model eliminates the overhead of repeated bootstrapping and ensures accountability of the vehicles for incident reporting and other critical actions. The two main features of our framework, viz. a three- party authentication and privacy protocol, and a trust propagation model, both of which are crucial for a successful deployment of VNs, can also be used in any generic security application.