# Early Gapping and Platoon Merging Strategies for Autonomous Vehicles using Local Controllers
## 4 Discussion  
To allow better management of merging AVs, it is suggested to impose gap profiles by using RSUs along the road. These ensure smooth merging and optimize vehicle flow rates. It is desirable for the merging vehicles to adapt their speeds and gaps to the main lane vehicles as much as possible to reduce negative traffic impacts due to changes in vehicle speed or gaps in the main lane. Therefore, a merge control algorithm and an adjustment of the ramp vehicles' approaching speed and gap are required. In this work, we explore the effect of gapping strategies on merging success, mainly in cases of high inlet density, using a numerical simulation of a main lane and a merging lane with vehicles driving at 100 km/h. About 140,000 different cases were run to cover large variations in the gapping conditions and to eliminate random effects. In all cases, the vehicles in the main lane were of priority. As the platoon sizes became larger, higher flow rates were obtained. Larger sizes of merging platoons allowed better merging success. Longer preparation and stabilization sections allowed larger merging percentages. The merging success was also influenced by the gapping speed \(v_{gapping}\) (i.e., allowing for vehicle \(i\) to start increasing the gap when vehicle \(i-1\) is at a lower speed than the speed limit). For example, for \(x_{merge}\) = 1.5 km and \(\%d_{stb}\) = 90%, changing \(v_{gapping}\) from 100 km/h to 75 km/h increases the merging success from 20% to 75%. However, a value of \(v_{gapping}\) lower than 75 km/h can result in a braking shockwave that propagates upstream and should be further investigated in future work (Figure 6). Figure 7: Effect of platoon sizes of merging and main lanes on: (**a**) obtained traffic flow [vel./h]; (**b**) merging success (%); (**c**) maximal time duration [min] along the section. The suggested method may control the merging rate of up to 23% during rush hour (Figure 4). Based on mass conservation, the inevitable consequences of high merging rates are delaying the vehicles on the mainline, as evidenced by the mainline trajectories (Figure 7c) taking gentler slopes with an average speed of 20-30 km/h along the preparation section (for 2-3 min) and then accelerating back to 100 km/h prior to merging. This approach allows for control of the merging percentage while setting a predefined moderate average speed along the preparation section and high speeds prior to merging. Indeed, when inlet density in the main lane is lower, higher merging success can be achieved (see Figure 4), similar to merging rates achieved using the zipper method [13,14]. However, in our case, vehicles are set to drive at moderate speeds (without full stops), resulting in substantially higher flow rates of more than 2700 veh/h, compared to the reported flow of about 1000 veh/h in the zipper method [36]. In cases which occur more often, when the merging flow is at a lower density comparable to the freeway mainline flow, gap profiles can be adjusted to allow lower merging demand to avoid the underutilization of capacity. The zipper method is considered fair; however, it may be sensitive to fluctuations and therefore can result in non-smooth mergers and phantom jams, which reduce the resulting flow. Moreover, when considering higher inlet densities, it results in a traffic jam [37]. In contrast with the zipper method, where vehicles interlace uniformly along the line, we suggest platoon merging, where dense clusters of vehicles merge into large gaps. Higher flow rates can be achieved with preparation sections of >0.4 km (Figure 6). If the desired merging percentage is higher, then the preparation section should be increased above 1 km before the merging site. For such a case, larger stabilization sections result in higher merging percentages and more incidences of abrupt maneuvers.