# A Comprehensive Real-Time Traffic Map for Geographic Routing in VANETs
## 1 Introduction  
Due to advances in wireless communication technologies, traditional wired data transmissions have been gradually replaced by wireless transmissions. The technologies have been extensively applied to all categories of mobile devices, such as smartphones, tablets, and wearable devices, to better facilitate people's daily lives [1]. In recent years, vehicles on the move can also enjoy the convenience of wireless communication technologies by assisting each other in message exchange and form an interconnecting network, namely Vehicular Ad Hoc Networks (VANETs) [2]. In a VANET, each vehicle is capable of communicating with nearby vehicles and accessing information provided by the network. With the appropriate application systems, drivers can receive warning messages when approaching an accident site to ensure driving safety. Moreover, real-time traffic information can also be used to avoid traffic jams. Many other applications of VANETs have been developed [3]. In addition, VANETs have been regarded as one of the major technologies to support most future Intelligent Transportation System (ITS) [4]. Communications in VANETs can be classified into two categories [5; 6]: Vehicle-to-Infrastructure (V2I) and Vehicle-to-Vehicle (V2V) (Figure 1). In V2I communication, vehicles make connections with infrastructures located at roadsides, namely roadside units (RSUs), to connect to the Internet. The fixed infrastructures act as gateways in data transmission. To provide a large service range, low transmission latency, and high throughput, cellular network (CN) technologies, such as LTE and WAVE, are usually adopted in V2I. Infrastructure deployment costs are the main drawback of V2I. On the other hand, V2V allows vehicles to form a self-organized network without any centralized system and fixed infrastructures. Due to the relative maturity of CN and the lower deployment cost of V2V, this has become a major trend of researching VANETs. Although V2V communications in VANETs are sometimes considered as a subclass of the Mobile Ad Hoc Networks (MANETs), there are many differences between those two network paradigms. First, mobile nodes in VANETs (vehicles) usually move at a higher speed, which results in a highly dynamic topology. Second, the nodes in VANETs are moving on pre-defined roads, rather than the random mobility assumption in MANETs. Besides, VANET nodes are usually assumed to have less limitation on battery and computing power. Those differences cause different network behaviors and characteristics, which make the solutions designed for MANETs not directly applicable to VANETs, e.g., routing protocols. Routing is a fundamental network function in both MANETs and VANETs. Its object is to relay packets from a source node to a destination node in a multi-hop manner. The early MANETs studies [7; 8] focused on the topology-based routing protocols which either proactively or relatively updated information used in routing decisions. When applying the topology-based protocols in VANETs, it has been shown as a high overhead and a low throughput due to the rapid topology change [9]. Numerous studies [10; 11; 12; 13] have proposed geographic routing protocols, known as position-based routing protocols, for VANETs routing. Each vehicle is assumed to be equipped with a positioning device, i.e., GPS (Global Positioning System), to obtain its own geographical location. Periodical beacon messages are broadcasted in a neighborhood to announce vehicles' identities, locations, speeds, and relevant driving information. Rather than finding an entire delivery path, geographic routing selects the most appropriate next-hop relay node, e.g., the neighbor closest to the destination, and forwards the packets to it. Many geographic routing protocols [11; 12; 14] in an urban environment utilize traffic information, i.e., the real-time vehicular densities of roads, and those with higher traffic flow are preferred so as to avoid disconnectivity along the forwarding path as well as high carry delay.