Vehicular ad-hoc networks is an emerging research area focussing on communication infrastructures that support vehicles and road-signs in distributing roadstate data such as information about hazardous road conditions ahead, approaching emergency vehicles, and traffic delays. Vehicular ad-hoc networks combine the areas of sensor networks (data acquisition) with mobile ad-hoc networks (highly dynamic topology and lack of pre-existing infrastructure). One of the main challenges of vehicular ad-hoc networks is the data dissemination protocols capable of distributing road-state information among vehicles. This paper presents two candidates for dissemination protocols: a zone flooding protocol and a zone diffusion protocol. The two protocols combine ideas from sensor networks and geocasting to ensure that data is aggregated and distributed only in a bounded geographical area. We present a comparative simulation study of the two protocols evaluating their relative performance using conventional metrics (such as network load) as well as application-specific metrics (such as awareness). The simulation study has been conducted using the Network Simulator 2 (NS-2) and has highlighted key properties of the two protocols that can be used as a basis for selecting the most appropriate protocol.