## 2 Related Work

Table 1: Comparison of existing clustering mechanisms [22; 23; 24; 25; 26; 27; 28] and our framework. station. Nevertheless, the mobility patterns of a fleet following a common route are not considered in the clustering and gateway management. [PERSON] et al. [24; 25] dynamically formed warning groups on the road to prevent rear-end collisions through vehicle-to-vehicle communications. A warning group is formed by a sequence of vehicles in the same lane such that each vehicle does not keep a safety distance from the vehicle in its front except the first one. When any vehicle of a group takes an emergency brake, a warning message will be sent immediately to those vehicles behind the braking vehicle in the same group. The warning messages may be delivered through multi-hop forwarding with the highest priority. Thus, drivers can become aware of such emergency braking event even before they actually see the braking signals. [PERSON] et al. [26] used the Long Term Evolution (LTE) network as the fixed infrastructure to centrally group vehicles into single-hop clusters within at most the communication range of IEEE 802.11p, whereas our proposed framework can group vehicles into multi-hop clusters in a distributed manner. In [26], the base station tries to create clusters with a lifetime as long as possible. In each cluster, a cluster header is elected, which has the responsibility to send its floating car data (e.g., position, speed, and direction) and its cluster members to the base station via LTE. Cluster members send their floating car data only via 802.11p to the cluster header that aggregates the floating data of cluster members before sending the floating data to the base station. In particular, the cluster head can avoid sending the useless data of a vehicle with the unchanged heading and velocity to the base station. Thus, LTE bandwidth usage can be reduced by alleviating redundant data transmissions. In addition, a compression algorithm can be used on the aggregated data to further save LTE bandwidth. [PERSON] et al. [27] employed the dedicated short range communications (DSRC) based on the IEEE 802.11p PHY and MAC layers to enable vehicle-to-vehicle communications. DSRC utilizes 75 MHz bandwidth from 5.850 to 5.925 GHz that is divided into seven channels, where one of the channels is the control channel for safety applications and the remaining six ones are service channels for non-safety applications. The speed difference between neighboring vehicles is considered for constructing a stable clustering structure. Stable and unstable clustering neighbors are defined according to their speeds and relative moving directions. Clusters are formed only among stable neighbors such that vehicles show similar mobility patterns in one cluster. Thus, fast moving vehicles are grouped on the fast speed lanes in one cluster while slow moving vehicles are grouped in another cluster, which can increase the cluster lifetime and reduce the transition of vehicles between clusters. [PERSON] et al. [28] analyzed the event-driven warning message propagation process (i.e., detecting an accident) to suggest a suitable message lifetime for guaranteeing the all relevant vehicles (in the zone of relevance) can be notified before reaching the potential danger. The analytical models are proposed to investigate the propagation process in a connected network (where all relevant nodes are in the same group) and a partitioned network (where the relevant nodes are in different groups). In addition, the delivery success probability of warning messages is derived for different traffic conditions. Moreover, a direction-aware broadcast protocol is proposed to reduce warning message redundancy during multi-hop broadcast in the partitioned network.