## 4 Test Protocol for Empirical Studies
### Procedure and Instruction
#### 4.3.2 Testing Process of HMI Outside the SAV  
For the application scenarios of HMI outside the bus, more attention should be paid to whether the external interacting objects can better understand the driving intentions of the bus through HMI. The interacting objects here refer to pedestrians, other autonomous vehicles, motor vehicles, non-motor vehicles, etc. When the driving direction of the bus does not conflict with the external interacting objects, the application of AD technology on the bus will not have a substantial impact, and the content displayed by HMI will not affect the user's acceptance of this technology. Therefore, the experimental design of eHMI will focus on the three conflict scenarios mentioned in Section 3.2.3 and ensure that the tests are conducted in a traffic environment without right-of-way rules, as at this point, eHMI is the sole means for users to receive information, aiding in variable control. The experiment selects pedestrians outside the vehicle as the perspective for testing the user experience, which can be divided into two processes. In the first process, the primary objective is to assess user acceptance levels when operating a conventional vehicle under human control instead of an SAV equipped with eHMI. In contrast, the second process aims to convey information to pedestrians through visual technology, thereby enabling a comparison of user acceptance across the selected eHMI concepts. The whole experiment should be conducted in a controlled environment with a trackable area to ensure the participant's safety and range of movement.  
The first process of the experiment should include specific tasks from A to E, as illustrated in Figure 12. These tasks are presented in a randomized order, with each task separated by a set time and distance to maintain experimental randomness.  
1. A to B: In a virtual environment, participants start at a one-way street and are instructed to cross to point B on the opposite side. AVs or conventional vehicles pass  
Figure 12: Schematic diagram of the specific scenarios and tasks of the entire test.  
uninterruptedly from the side, and participants use the information displayed on the eHMI to decide when to cross.
2. B to C: When approaching a vehicle from a rear diagonal position, participants assess its type (AV or conventional) and status using the HMI displayed on the vehicle, enabling them to navigate the intersection safely.
3. D to E: Participants encounter an AV or conventional vehicle from the opposite side. Due to road construction, only one party can pass through first from the narrow gap, and the eHMI provides information on which party has priority.  
The second phase of the experiment, focusing on the A-to-B step in Figure 12, involved instructing participants to repeatedly cross the intersection between points A and B via the pedestrian crosswalk, facing only Avs (see Figure 13). The number of repetitions corresponded to the different eHMI concepts being tested. To mitigate potential order effects and balance experimental conditions, a randomized Latin square design was implemented to determine participant assignments.