  The dynamic nature of vehicular ad hoc network (VANET) induced by frequent
topology changes and node mobility, imposes critical challenges for vehicular
communications. Aggravated by the high volume of information dissemination
among vehicles over limited bandwidth, the topological dynamics of VANET causes
congestion in the communication channel, which is the primary cause of problems
such as message drop, delay, and degraded quality of service. To mitigate these
problems, congestion detection, and control techniques are needed to be
incorporated in a vehicular network. Congestion control approaches can be
either open-loop or closed loop based on pre-congestion or post congestion
strategies. We present a general architecture of vehicular communication in
urban and highway environment as well as a state-of-the-art survey of recent
congestion detection and control techniques. We also identify the drawbacks of
existing approaches and classify them according to different hierarchical
schemes. Through an extensive literature review, we recommend solution
approaches and future directions for handling congestion in vehicular
communications.
