## 2 Scenario  
In the proposed scenario, shown as Figure 1, the dotted line 3 is the trajectory of the oncoming vehicle and the dotted line 1 is the initial reference trajectory (also known as the global reference trajectory) of the ego vehicle. Due to the existence of the parked vehicle and oncoming vehicle, the ego vehicle needs to alternate the local trajectory planning (shown as the dotted line 2) at this stage. Considering the different initial position and the speed change in the process of the oncoming vehicle's motion, it may be not safe for the ego vehicle if it still maintains the current speed or accelerates within the allowable speed boundaries or handling stability requirements. Therefore, it is inevitable for the ego vehicles to decelerate in order to handle the driving situation with guaranteed safety. Considering the dynamic obstacles like oncoming vehicles in the opposite lane, especially when future speed of the obstacle is unpredictable, it is difficult for the controlled ego vehicle to behave in a similar way of judgment as a skillful and careful human driver. Therefore, how to re-plan the motion behavior and local reference trajectory is a question worth pondering. Before introducing a collision avoidance strategy, it is necessary to refine the scenario and establish some restrictions. This research aims to propose an upper-level control algorithm to improve the safe avoidance of unpredictable obstacles. By default, the relative position relationship between the vehicles and the real-time speed of the oncoming vehicle can be accurately measured by automotive radar. As shown in Figure 2, A represent the ego vehicle, B is the parked vehicle and C is the oncoming vehicle. A1, C1 are the positions when the ego vehicle and the oncoming vehicle meet with each other (the same for A2, C2). \(L_{a}\) and \(L_{b}\) are defined as the distance between the ego vehicle and the front/rear side of the parked vehicle, respectively. \(L_{c}\) is defined as the distance between the oncoming vehicle and the front side of the parked vehicle. \(D_{safety}\) is a safe distance which is used as a reference for speed control of the ego vehicle. Unless otherwise stated, the units of measurement for distances is meter in this paper. We consider that:  
1. \(L_{a}\), \(L_{b}\) are global fixed parameters for the scenario, that means the initial relative position of the ego vehicle and the parked vehicle are not changed in any simulation;
2. \(L_{c}\) is treated as a local invariant, that means the initial relative position of the ego vehicle and the parked vehicle will only be changed as the initial condition of a simulation;
3. \(0\;\mathrm{m/s}\leq v_{a}\leq 15\;\mathrm{m/s}\), \(10\;\mathrm{m/s}\leq v_{c}\leq 15\;\mathrm{m/s}\);
4. \(-3\;\mathrm{m/s^{2}}\leq a_{sc}\leq 1\;\mathrm{m/s^{2}}\), \(-3\;\mathrm{m/s^{2}}\leq a_{OnC}\leq 1\;\mathrm{m/s^{2}}\). Figure 1: Target scenario of collision avoidance overtaking a parked vehicle and yielding an oncoming vehicle. Where \(v_{a}\), \(a_{sc}\) represent the speed and acceleration/deceleration of the ego vehicle respectively. \(v_{c}\), \(a_{OnC}\) represent the speed and acceleration/deceleration of the oncoming vehicle respectively. \(D_{safety}\) is a distance used to determine a safe passing position. It is obvious that such a scene often occurs in crowded urban traffic conditions. The behavior of the ego vehicle will be re-planned by the calculated passing position, shown as follows:  
1. If the passing position is far enough away from the parked vehicle, as shown in Figure 2a at passing position 1, the ego vehicle will choose to keep speed unchanged or accelerate according to the oncoming vehicle speed \(v_{c1}\) and position \(L_{c1}\);
2. Due to excessive speed \(v_{c1}\) or short initial distance \(L_{c2}\) of the oncoming vehicle, even if the ego vehicle accelerates to the maximum allowable speed, it cannot guarantee that their passing position is at a safe position, as shown in Figure 2b at passing position 2.