## 6 Discussion and Conclusions  
The PEEV-WS, i.e., the emergency vehicle warning system deploying the VANET network, was proposed and implemented using the OMNeT++ network simulator and assessed by computer simulations. To facilitate the interpretation of the system messages, and to retain the privacy of EV's dynamic data, network layer routing was selected as the message forwarding approach for the system's messages. The performance of the three candidate ad hoc routing protocols (namely AODV, GPSR, DYMO) and the reaction time the PEEV-WS provides to drivers, were evaluated using realistic scenarios. The simulation results suggest that the AODV protocol can support the communication generated by the PEEV-WS. The proposed system deploying the AODV network routing protocol complies with the latency requirements for the high-latency and low-frequency warning applications [32], in almost all situations. The average end-to-end delay exceeded 1000 milliseconds only for the dense urban network scenario when the EV transmitted messages to distant vehicles. In these cases, the reaction time exceeds 100 s, hence, larger delays do not affect the functionality of the system. Figure 8: The available reaction time as a function of the distance between the EV and the vehicle, in (**a**) the \(\hat{\text{Z}}\)lina scenario and in (**b**) the Bratislava scenario. According to Emergency Medical Management Cooperative (EMMCO) West [46], the effective range of EV audio warning systems tops at less than 460 m in ideal conditions. When the environment is noisy and the vehicle's windows are closed, the reaction time can be as low as 4 s. The proposed EV warning system can increase reaction time significantly. Thanks to the larger communication range, which reached up to 1750 m, the maximum available reaction time reached 320 s. High values of available reaction time open a new set of issues that have to be addressed as there is no point for the driver to take any action for several minutes. On the contrary, such an action could disrupt traffic fluency and cause other unexpected effects. Therefore, especially when the available reaction time is large, the EV should update the vehicles with information about the estimated time when it is supposed to reach the informed vehicle. Hence, the methods providing predictions of the EV's driving time should be used in a V2X-based emergency vehicle warning system [47]. On one hand, the simulation results suggest that the proposed warning system is feasible. On the other hand, if the PEEV-WS is deployed in the real world, we can expect many more situations and variables that could affect the performance of the system, e.g., different traffic conditions, buildings or other constructions potentially blocking signal propagation, different routing protocols, etc. Consequently, the parameter and state spaces are huge and could not be fully explored in this paper. It is worth noting here that the goals of deploying the V2X-based EV warning system are twofold: (i) to assist formation of an emergency lane free of obstacles; and (ii) to provide assistance in keeping the formed emergency lane clear. The system proposed in this paper covers the first aspect, while the second is left for future work. Furthermore, future work should extend the PEEV-WS to more complex use cases that go beyond the emergency lane. Another great research challenge is the transition period with a presence of mixed traffic (conventional and connected vehicles). Finally, emerging technologies with a high potential for reliable low-latency communication, in particular, 5G and vehicle-to-infrastructure communication, could be used to disseminate EV warning messages to the vehicles and connected infrastructure, e.g., traffic lights and variable message signs. The communication could be managed either via the traffic management center or locally, which will bring new challenges regarding communication latency as well as system security and safety. Conceptualization, T.P., P.P., L.B., and M.D. ; methodology, T.P., P.P., J.R., L.B., and M.D.