## 3 Implementation of Vehicular VLC System
### Proposed Mechanism of Data Exchange in a VLC-Based Chain Network in the Tunnel  
We describe a mechanism of data exchange between vehicles driving within a tunnel via the considered VLC-based chain network, which can help to mitigate chain collisions and hard braking incidents in tunnel environments. To facilitate understanding, we consider the proposed VLC-based chain network involving three vehicles (i.e., leading, middle, and trailing vehicles) within a tunnel. The leading vehicle measures its target speed and acceleration and transmits the measured data to the middle vehicle. After receiving the data, the middle vehicle compares the incoming acceleration with its own. If the received acceleration is greater, which suggests that the leading vehicle is accelerating faster, the middle vehicle forwards the received data to the trailing vehicle. On the other hand, if the middle vehicle's acceleration is higher, it sends its own acceleration data to the trailing vehicle. Once the trailing vehicle receives the data from the middle vehicle, it compares the received acceleration to its own and continues communication between the vehicles. When the distance between consecutive vehicles increases beyond the effective VLC communication range, the communication link naturally becomes unavailable. However, in such cases, this is interpreted as an indirect indicator of low collision risk: a vehicle that is sufficiently far from its predecessor has more time and space to respond safely, thus reducing the urgency of real-time coordination. Accordingly, in our system logic, if a vehicle fails to receive data, it transitions into a leading-role behavior and begins broadcasting its own data to downstream vehicles. The process of data exchange between vehicles in the considered network can be illustrated as a flow chart, as shown in Figure 6.  
Figure 5: Brief Circuit for the integration of the proposed vehicular VLC system with the LED tail-lights of actual vehicles. Red lines indicate the TX data paths into the NE555 trigger and reset, whose output drives the LED via an NPN transistor. Figure 6: Flow chart for the data exchange between vehicles within a tunnel via the considered VLC-based chain network. Table 1 compares the proposed vehicular VLC system, the vehicular VLC system and controller area network (CAN) bus interface [20], the existing vehicular optical wireless communication system [26], and RF-based tunnel communication systems [27], focusing on cost, communication method, and scalability. The proposed vehicular VLC system reuses each vehicle's existing LED lamps as the transmitter and receiver. Also, the proposed vehicular VLC system is implemented using low-cost MCU hardware (e.g., Arduino Mega), making it extremely economical. By maintaining an LOS V2V chain topology, it can relay data between vehicles without any tunnel-mounted infrastructure, ensuring reliable in-tunnel communication. In contrast, the vehicular VLC system and CAN bus interface require expensive equipment for their implementation (e.g., USRP-2922) and their performance may degrade due to NLOS. Additionally, the vehicular optical wireless communication system requires custom LED overdrive modules, an expensive photodetector (e.g., PDA100A2), and additional optical components (band-pass filter and lens), resulting in high implementation costs. Vehicular optical wireless communication systems support only point-to-point (P2P) links and the performance degrades under non-line of sight (NLOS) conditions. The existing optical wireless V2V communication system has not yet been applied to real vehicles. However, RF-based tunnel communication incurs substantial costs for on-board units (OBUs) and roadside units (RSUs), requires space for RSU deployment and backhaul cabling, and suffers from scalability bottlenecks when many OBUs share a single RSU.