# UAV-Assisted Hybrid Scheme for Urban Road Safety Based on VANETs
## 3 UAV Assisted (UAVa) Reactive Protocol Strategy
### UAVa's Design Approach
#### 3.2.1 Uava's Network Architecture  
UAV's network architecture is shown in Figure 2. Here we represent different kinds of scenarios, road segment nodes, and road intersection allocated RSUs. In an emergency basis, such as sudden congestions or blockage, UAVs assist in RSU at road intersections to increase the RSU's coverage and capacity to regulate the situation. We consider that every node is equipped with an on-board unit (OBU), which aims to achieve V2V communication with other nodes on the road segments and V2R and V2U connection with RSU and UAV in the road intersections. OBU records overall information of a node, such as nodes ID, position, current-time speed, etc. It obtains details on the subsequent road intersection from the RSU through V2R communications. The source node can calculate the travel time based on the current-time speed of the road segment and record it by its OBU. If any blockage occurs on a particular road intersection, such as sudden traffic accidents or congestion, OBU will produce a warning message and send it to RSU, and RSU will relay this warning message through the entire traffic network, which contains the ID of the intersection and the approximated time to release the congestion. In VANETs, V2V communication efficiency will be influenced by the speed of linked nodes in the road segments. Different V2V communication mechanisms are used to collect and exchange current-time traffic information according to different requirements of all the connected nodes in the road network. Suppose a large number of linked nodes are installed on the road network, then the current-time traffic information can be exchanged via V2V communication with multi-hop services. Due to the huge amount of connected nodes in the road segments, the source node can find reliable relay nodes within its wireless communication range, and one hop can send current-time traffic data to the relay node. Current traffic data can be transmitted along with the road segments until at the intersection the relay node joins the wireless communication range of the RSU. Using V2R or V2U communications, the RSU or UAV at the intersection may collect current-time traffic information while  
Figure 2: Transportation network architecture. the large majority of connected nodes on the route, the effect of data transmission delay on travel time estimation accuracy, are typically ignored. When a small part of connected nodes is deployed in a vehicular ad hoc network, current-time traffic information on the road intersections can be accumulated and shared through V2R communication. Due to the low rate of connected nodes, the current-time traffic information accumulated and shared along with the road segments, via the carry-and-forward mechanism, suppose a source node within its wireless communication range cannot find a reliable relay node to share the current-time traffic information. In this scenario, the node will continue to move along the path with current-time traffic information. If a stable relay node is located by the source node, the information will be transmitted by continuing to switch between forward and backward until the relay node reaches the contact range of the RSU at the intersection. The RSU uniformly deployed each intersection to accumulate the current-time traffic information from the mobile nodes through V2R or V2U communications in VATENTS. In the entire network, the RSUs of the road network are connected through a cellular network, and the R2R communication is realized through our designed current-time traffic data accumulation and sharing mechanism. We considered two types of the traffic data being stored in the memory of RSU, which is the static data type and the dynamic data type. Static data contain the distance of each road segment, road intersection information, the capability of different areas, all of which are obtained through digital maps.