I. Introduction
Vehicular ad hoc networks (VANETs) can be applied in reducing traffic accidents, improving traffic efficiency and management and many other fields [1]. VANETs are mainly composed of vehicle-to-vehicle (V2V) [2] network and the vehicle-to-infrastructure (V2I) [3] network, where the Road Side Unit (RSU) acts as an Internet access point to send data to the data center (DC) or cloud [4]. There are many research areas of VANETs, among all of which traffic density estimation is most widely studied [5]. Traffic density is essential and necessary to many applications like mobility-centric data dissemination [6] and other routing mechanisms [7]. Moreover, higher traffic density means higher connectivity, which facilitates the V2V communication. As a result, traffic density information is favorable to V2V related applications. In addition, a density distribution map can be used to estimate the pollution status in different places and optimize the deployment of VANETs infrastructure [8]. Therefore, it is rather meaningful to estimate traffic density.