Safety applications are the cornerstone of the envisioned Vehicle Ad Hoc Network (VANET). Early transmission of alert messages following car accidents can avoid further potential crashes and save lives. However, the limited terrestrial coverage on highways, particularly in rural areas with low traffic density, hinders the deployment of this service. One promising solution involves integrating Unmanned Aerial Vehicles (UAVs), commonly known as drones, into VANETs to serve as flying relays. These UAVs can re-broadcast alert or warning messages between vehicles, bridging communication gaps. In this paper, we propose a unified UAVs-VANET architecture where UAVs relay messages among vehicles on rural highways. To evaluate our approach, we consider a case study involving a road accident on a Tunisian highway (Tunis - Bou Salem 75 km). We use the SUMO simulator in conjunction with NS3. The obtained results showed that 100% of vehicles are alerted within 3.85 seconds. The study also investigates how the number of deployed UAVs impacts the number of alerted vehicles.