Vehicular Ad Hoc Networks (VANETs) is an emerging area of research and have been gaining significant attention over recent years due to its role in designing intelligent transportation system. It includes vehicle-to-vehicle (V2V) and vehicle-to-infrastructure (V2I) message flows, supported by wireless access technology such as, IEEE 1609 WAVE and IEEE 802.11p. One of the major scientific challenge in VANET implementation, is the design of routing protocol that could provide efficient and reliable node-to-node packet transmission. Routing in VANETs is a complex task in urban environment. This paper reports the overall performance evaluation of two existing routing protocols namely, Ad hoc On-Demand Distance Vector (AODV) and Dynamic Source Routing (DSR) for VANETs. This study aims at optimizing the selection of best possible routing protocol for providing reliability to data packet dissemination in an efficient way. The impact and effectiveness of existing topology based routing protocol for VANETs application has been evaluated through the use of NetSim software tool. The simulated results shows that a combination of proper channel model together with an efficient routing protocol enhance the link throughput of the VANET for a fixed network size. Further, performance evaluation also demonstrate the impact of network sizes and routing protocols on packet loss, packet delivery ratio, average end-to-end delay and overhead transmission.