# Development of Multimodal Physical and Virtual Traffic Reality Simulation System
## 5 Case Studies
#### 5.1.2 Experimental Procedure  
The experiment was conducted in a controlled virtual environment where a human driver passed a bicyclist riding in a designated bicycle lane. At the beginning of each trial, the driver started approximately 100 m behind the bicyclist to ensure that the bicyclist was unaware of the approaching vehicle until it entered their peripheral vision. This setup was designed to replicate real-world scenarios where bicyclists experience unexpected vehicle interactions.  
Once within proximity, the driver continued to approach and eventually passed the bicyclist while maintaining a consistent speed and lateral position. To ensure experimental consistency, the same driver was used across all trials, while the bicyclists varied. Each participant experienced only one passing event per trial to capture an unbiased physiological and behavioral response without the influence of repeated exposure.  
To demonstrate the flexible design of our simulator setup, we incorporated two different visualization methods for participants. The bicyclist experienced the virtual environment using a VR headset (VIVE Pro), providing a fully immersive perspective of the passing event. Meanwhile, the driver used the standard simulator setup with three 55-inch TV screens, ensuring a realistic but conventional driving experience. The perspectives and experimental setup are illustrated in Figure 6.  
Data were collected from two primary sources:  
* Position and speed data of both the driver and the bicyclist, recorded at a frequency of 0.1 s. This provided detailed insights into movement dynamics, including lateral positioning and vehicle-bicycle proximity during the passing event.
* Bicyclist heart rate measurements, recorded every 1 s, capturing physiological responses to vehicle proximity.  
To analyze how bicyclists responded to passing vehicles, we identified a key reference point in each trial, referred to as the critical moment. This was defined as the time at which the vehicle reached its closest lateral distance to the bicyclist, referred to as the critical distance. The critical distance represents the minimum separation between the vehicle and the cyclist during the passing event.  
As shown in Figure 7, the critical moment anchors the analysis, enabling us to examine both the behavioral and physiological responses before and after this key point. Specifically, we computed the following:  
* Average lateral position of the bicyclist during the 5 s before and after the critical moment to determine whether participants adjusted their positioning in response to the vehicle. The lateral position was measured relative to the bicycle lane, with the left edge of the lane defined as 0 m and the right edge as 2 m. This provided a standardized reference for assessing lane adjustments.
* Average heart rate during the 5 s before and after the critical moment to assess potential stress responses.  
Figure 6: Participants’ perspectives in Case Study I.  
By structuring the analysis around this critical moment, we aimed to quantify how bicyclists dynamically adapted to passing vehicles, offering valuable insights into defensive cycling behaviors and potential physiological stress reactions.