Vehicular networks are emerging as a new distributed system environment with myriad possible applications. Most studies on vehicular networks are carried out via simulation, given the logistical and economical problems with largescale deployments. This paper investigates the impact of realistic radio propagation settings on the evaluation of VANET-based systems. Using a set of instrumented cars, we collected IEEE 802.11b signal propagation measurements between vehicles in a variety of urban and suburban environments. We found that signal propagation between vehicles varies in different settings, especially between lineof-sight (“down the block”) and non line-of-sight (“around the corner”) communication in the same setting. Using a probabilistic shadowing model, we evaluate the impact of different parameter settings on the performance of an epidemic data dissemination protocol and discuss the implications of our findings. We also suggest a variation of a basic signal propagation model that incorporates additional realism without sacrificing scalability by taking advantage of environmental information, including node locations and street information. 1. Introduction Vehicular networks are emerging as a new distributed system environment with myriad possible applications that range from traffic information systems and road safety [1]– [5] to urban sensing and entertainment [6]. Vehicular adhoc networks (V ANETs) provide infrastructureless, rapidly deployable, self-configurable network connectivity. The network is made of vehicles interconnected by wireless links and willing to store and forward data for their peers. As vehicles move freely and organize themselves arbitrarily, message routing is done dynamically based on network connectivity. Compared with other ad-hoc networks, V ANETs are particularly challenging due in part to the vehicles’ high rate of mobility and the numerous signal-weakening obstructions, such as buildings, in their environments.Because