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 large-scale 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 line-of-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.