Congestion in vehicular ad hoc networks affects the performance of delay-sensitive applications when exchanging emergency or general information sharing messages. In particular, during emergencies on roads like road accidents and security warnings demand high reliability and low latency. However, in traditional solutions, such messages use the same control channel for transmission leading up to a saturated or congested channel. Furthermore, the highly dynamic nature of vehicular networks

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... up to unpredictable routing patterns, subsequently degrading the network performance. An uneven deliverance of these critical messages can be catastrophic for the delay-sensitive applications. Thus, congestion control remains one of the most challenging problems within the domain of vehicular networks. This paper provides a comprehensive overview of the working of vehicular networks, the recent research advances to cater for congestion in such networks, and open problems and challenges relevant to congestion avoidance. INDEX TERMS Ad hoc vehicular network, congestion avoidance, power-based, rate-based, priority-based, clustering-based, CSMA/CA-based, hybrid, vehicular simulation. NOMENCLATURE ACRONYMS