# A Comprehensive Real-Time Traffic Map for Geographic Routing in VANETs
## 1 Introduction
Recently, several creative real-time traffic collection systems based on vehicles have been proposed. In studies [15; 16], accelerometers in mobile phones of drivers were utilized for motion detection and traffic sensing. In [17], a larger-scale information collection system aimed to reduce the collection, fusion, and dissemination time was proposed. However, most of the existing routing protocols do not describe how to gather real-time traffic. They assume this information is either already available or can be obtained by querying an existing traffic center. Query delay and the infrastructure construction cost are ignored in those works, but some [18; 19] do notice this issue. An Infrastructure-Free Traffic Information System (IFITS) is proposed in [18]. In an IFITS, a road segment is divided into fixed-size cells, and the group leader of each cell is responsible to calculate the number of vehicles in the cell. The total number of road vehicles is then accumulated by the group leader of the last cell on the road. In [19], a probe  
Figure 1: Different communication models in Vehicular Ad Hoc Networks (VANETs): (a) Vehicle-to-Infrastructure (V2I); and (b) Vehicle-to-Vehicle (V2V). message is used to estimate the connectivity of vehicles nearby road segments. A probe message is initiated by a node near an intersection and forwarded in the road segment. If the probe can reach the next intersection, a return message is sent to the original sender, which indicates that this road is traversable. Otherwise, the probe is dropped and the road is disconnected. This information is then used in route selection, which has been shown to improve the network throughput and delivery ratio. However, the aforementioned can accrue an extra message exchange cost and delay. Furthermore, only the traffic density in a small region can be cognizant. A wide-ranging and up-to-date traffic information is still omitted, which may lead to a wrong routing decision. In this paper, we propose a Comprehensive Real-Time Traffic Map, namely a CRT Map, to collect the wide-ranging real-time traffic information, based on which we then design a geographic routing protocol, called the CRT Map Based Routing (CBR). In the design of a CRT Map, the concept of Crowdsensing [20] is adopted. Vehicles share local views of traffic on individual roads and assist each other in constructing an overview of the whole road network. When driving on each road, a vehicle calculates how many cars it meets and then constructs a local traffic map. Then, local maps of different vehicles are exchanged at intersections and merge into their own maps. Consequently, each vehicle gradually has a much larger view of the entire network. In addition, to prevent out-of-date information, we take the effect of time decay into account when merging traffic maps. A CRT Map is then used in the proposed CBR. It extracts traffic information from a CRT Map and considers the connectivity of consecutive roads to select a reliable path in the routing decision. Compared with existing researches, the proposed solution has no extra delay, since a local CRT Map is maintained. Beacons are utilized in vehicle calculations, and, thus, map exchanges are the only extra cost. Exchange frequency is controllable in our design, which reflects a trade-off between accuracy and overhead. Simulation results show that a CBR can lower the end-to-end delay and increase the packet delivery ratio with a low overhead. The rest of this paper is organized as follows. In Section 2, the geographic routing protocols in VANETs are surveyed. Section 3 describes the system model, including the environment, assumptions, and the basic framework of the CRT Map. The information exchange mechanism of the CRT Map and the detail design of a CBR are presented in Section 4. In Section 5, we demonstrate the efficiency of the proposed schemes through simulation studies. Section 6 discusses future works and concludes this paper.