In a vehicular ad hoc network (VANET), highmobility and uneven distribution of vehicles are important factors affecting the performance of routing protocols. The high mobility may cause frequent changes of network topology, whereas the uneven distribution of vehicles may lead to routing failures due to network partition; even high density of vehicles may cause severe wireless channel contentions in an urban environment. In this paper, we propose a novel concept called the microtopology (MT),which consists of vehicles and wireless links among vehicles along a street as a basic component of routing paths and even the entire network topology.