## 3 Privacy Ensuring Emergency Vehicle Approaching Warning System  
To avoid the need to broadcast the EV's route, we propose a novel privacy preserving emergency vehicle warning system, which utilizes a unicast communication to target the selected vehicles only, i.e., those whose action is required to ensure fluent transit of the EV. The proposed application message format can be adopted by any other vehicle-to-vehicle technology allowing unicast communication. This makes the proposed system transmission technology agnostic. The proposed system assumes the following preconditions:  
* All the vehicles are equipped with the compatible communication modules and other hardware necessary to either display the information to a driver or act autonomously following the received requests. * All the regular vehicles except for the EV periodically disseminate the CAMs or other similar sources of data that can be used to estimate their current position at a lane-level resolution. * All the vehicles are equipped with the proposed emergency vehicle warning system deploying the standardized request codes, and drivers obey the requests provided by the system. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Parameter** & **Value** & **Requirement(s) Affecting the Choice** \\ \hline Message generation interval (\(\Delta\)t) & 1 s & Available reaction time \\ Communication range & \(\geq\)1000 m & Available reaction time \\ Carrier frequency & 5900 MHz & Interoperability, versatility \\ Channel bandwidth & 10 MHz & Interoperability, versatility \\ Physical and Medium Access & IEEE 802.11 & Interoperability, versatility \\ layers specification & Ad hoc routing (Ad hoc On-Demand & \\  & Distance Vector, Greedy Perimeter & Interoperability, versatility, \\ L3 and higher layer protocols & Stateless Routing, Dynamic MANET & security, information \\  & On-demand) + User Datagram & relevance \\  & Protocol (UDP) & \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Summary of the privacy ensuring emergency vehicle approaching warning system (PEEV-WS) specifications. In VANETs, the EV receives the CAMs [36] from other vehicles within its communication range. From the received data, the EV learns a topology of the network and extracts the information about other vehicles' locations and compiles a list of all reachable vehicles (see Figure 2). When the PEEV-WS is initiated, the main process executing the steps illustrated in Figure 3 is triggered. The main process updates the list of relevant vehicles based on CAMs. For each newly identified relevant vehicle, a communication handling process is triggered, and the communication process with already irrelevant vehicles is terminated. When an interference between EV trajectory and a vehicle is detected in the communication handling process, the corresponding warning message is created, and a routing protocol is used to find a communication route to the interfering vehicle (see Figure 3). This ensures that only the relevant vehicle receives the information about the approaching EV together with the specific request for the desired action. In general, to decide on a suitable action is a very complex problem and it is out of the scope of this paper. In simulations, we consider scenarios involving multiple lanes and the action is limited to changing lanes to one which is not interfering with the EV trajectory. Figure 2: Illustration of the emergency vehicle warning system operation. The EV collects vehicle locations from cooperative awareness messages (CAMs), decides on the needed actions, and sends requests to corresponding vehicles. In this example, vehicles V1 and V2 are obstacles for the EV in lane 1. V1 can be reached directly. V2 is not in the EV’s communication range, therefore a routing protocol is used to find a communication route to V2. Finally, the communication message is then sent from the EV, through V1 and V5, to its destination, V2. As communication between vehicles in VANET does not use acknowledgments, a sender does not have the information about the transmission result.