# Coordinated Multi-Platooning Planning for Resolving Sudden Congestion on Multi-Lane Freeways
## 5 Implementation and Experimental Results
Although the vehicles in Lane 0 leave the accident spot 7.24 s later than in the first scenario, the proposed platooning-based control still shows effectiveness in cutting down the waiting time significantly. In short, the vehicle-forming platoon can effectively extenuate the traffic congestion of sudden accidents. In addition, from Figures 6-8, we can observe that the platoon size affects the performance of the proposed CPP algorithm significantly. In our experiments, the best platoon size usually falls between 10 and 14. This observation aligns with consensus with the common setting of the platoon size in several empirical tests [20]. Figure 8: The mean time taken for the vehicles to exit the congestion spot if they receive an accident warning **1500** m ahead of the accident spot. Figure 9 shows the comparison of congestion delays of the last vehicle in each lane with the best platoon size setting under the three scenarios. We found that the congestion time could be reduced significantly if the vehicles received the accident warning at around a 3 km distance. Due to diverting the vehicles in Lane 1 and Lane 2 to the overtaking lane early, the vehicles in Lane 0 will have more safety conditions when leaving the blocked lane. According to the result, by increasing the distance by 1 km, the congestion time could be reduced by 5% on average. When the vehicle does not join any platoon, the vehicle can only receive an accident warning message at the warning spot. However, if a vehicle becomes a member of a platoon, it can receive the warning message earlier. Table 4 shows, on average, how far away and how early a vehicle can receive the warning message. The mean distance represents the average distance between the \(i\)th vehicle of the platoon and the platoon leader. The mean time indicates how early, on average, the \(i\)th vehicle of the platoon can receive the warning message. For example, the warning message is sent to vehicles within 1500 m. Table 4 shows that the 19 th vehicle (index 20) is, on average, 893.4 m behind the platoon leader (i.e., it is 2393.4 m away from the accident spot) and can receive the warning message, on average, 32.8 s earlier than vehicles that do not join a platoon. This distance complies with the transmission range of 5G V2V and Dedicated Short Range Communication (DSRC). Therefore, with an earlier warning, the vehicles will have more distance when seeking a safe lane-changing space without approaching the accident spot and slowing down. Table 3.0 _Cont._  
Since the accident only occurs in Lane 0, the vehicles in this lane are required for lane changing to exit the accident spot. In other words, even if the vehicles in Lane 1 and Lane 2 do not perform lane changing without a safe distance, they can pass through the accident spot successfully. Table 5 compares the average number of vehicles in Lane 1 and Lane 2 that do not perform lane changing under different distances of receiving the accident warning ahead of the accident spot. According to the results, we found that the earlier the warning to the vehicles approaching the accident spot fires, the faster the vehicle can finish platoon forming and exit the congestion area. Based on the above results, although a platoon with 20 vehicles [20; 26] is theoretically possible, it is very challenging to form such a long setting. First, maintaining the platoon becomes complicated if the vehicles are moving at a high velocity. Secondly, a small mistake or disorder in the lane change can cause a severe accident. We believe that there is a trade-off in the length of the platoon and the feasibility of the coordination schedule. We found that the average number of vehicles in a platoon for common traffic density is approximately 10 to 14 (see Figure 5) in order to commit a good balance for the scheduling efficiency, control complexity, and safety. Table 3.1 Comparison of the average reduction congestion delay when the vehicles receive the warning message at 3500 m, 2500 m, and 1500 m ahead.