## 1 Introduction  
Intelligent transport systems (ITSs) aim to reduce the number of accidents and deaths occurring on the roads every year. Such systems are advanced driver-assistance systems or autonomous vehicles equipped with sensors and actuators owing to which they can sense their environment and react accordingly. Communication networks are being studied as a way to propagate events between vehicle and roadside units (RSUs) and to support security or comfort applications. Sample applications include cooperative collision avoidance, platooning, and video streaming. Machine-to-machine (M2M) communications, in the form of vehicle-to-vehicle (V2V) and vehicle-to-infrastructure (V2I), grouped under the term vehicular ad hoc networks (VANETs), will benefit from 6G [1] to allow lower latencies and massively connected vehicles. The communication technologies envisioned for the future deployment of 6G, especially in ITSs, are still under development. Related reviews can be found in [2; 3]. In this study, we focused on current technologies and used them to evaluate proposed clustering strategies for these future communications. Researchers and industry have been working toward standardizing vehicular communication technologies supporting dedicated short-range communication (DSRC) [4] such as IEEE 802.11p [5] and IEEE 802.11 bd [6]. However, these technological solutions need to be supplemented with organizational models adapted to vehicle communication constraints. Indeed, vehicles can move freely along the road at various speeds, which causes frequent link failures and bandwidth degradation. In this context, the provision of quality of service (QoS) applications is expected to be challenging [7]. This has been an open research question, leading to the design of numerous network protocols on every layer of the Open Systems Interconnection (OSI) model. This study entirely focused on routing at the network layer, which is mainly in charge of routing and addressing functions. Several protocols for the network layer have been proposed in the literature to try to overcome network connectivity problems. One of the main approaches is clustering [8], for which nodes gather in virtual groups, thus allowing network functions such as routing [9] or transmission scheduling [10] to be centralized to an extent and reducing routing traffic overhead. This study assumed that the numerous clustering strategies [11] proposed in the literature can actually be divided into two groups:  
* Clustering strategies that lead to the formation of single-headed clusters connected through a backbone. The chain-branch-leaf (CBL) [12] clustering scheme is based on such a strategy. * Clustering strategies that consist, for each node, of selecting several relays in order to reach their _k-hop_ neighborhood, thus leading to a mesh network that covers every single node through several relays. The multipoint relaying (MPR) [13] technique is part of this group. Recent evaluations [14] showed that CBL--a backbone-like clustering scheme--outperforms most ad hoc routing protocols from the different approaches, namely, a reactive one such as the ad hoc on-demand distance vector (AODV) protocol, a proactive one such as the optimized link state routing (OLSR) [15] protocol, and a geographic one such as the geographic routing protocol (GRP). In addition to considerably reducing routing traffic overhead, the results showed that CBL achieves the lowest latency. However, the same work underlined that the single-headed clusters in CBL lead to a backbone structure that lacks redundancy, thus impeding alternative routes for V2V multihop communications in the case of link failure. The researchers also pointed out that CBL performance partly comes from its strategy, which consists of organizing V2V multihop communications per road traffic direction.