# An Augmented Warning System for Pedestrians: User Interface Design and Algorithm Development
## 2 Materials and Methods
### Interface Development  
Since the AR glasses can be worn by a pedestrian during normal walking in public, it is important to deactivate the warning system while no conflict is detected. Moreover, the AR interface should not be so complicated [19] that it interrupts the pedestrians perceiving the real-world information. Redundant information in the interface may increase the mental workload and the response time for the pedestrians when perceiving the warnings [20]. Thus, only necessary information should be shown in the warning interface. Since the warning system is designed for the pedestrian when their view is obstructed or situation awareness is low, the information the pedestrian cannot perceive, such as the direction and the expected path of the oncoming vehicle, the conflict point, as well as the time to the collision, should be shown to the pedestrian. With such information present, the pedestrian is able to identify the location of the oncoming vehicle and the level of the risk. Since there are no well-built guidelines for the AR interface design, the smartphone and the human-computer interface designs were used to match the top-down human perception  
Figure 1: Warning system summary. process [21]. A sidebar design was used to show the oncoming vehicle direction based on smartphone and human-computer interface designs [22; 23]. Moreover, general warning information design rules, such as color code [24] or a warning symbol, were adapted in the current design of the warning information to match the existing mental models for the general public [25; 26]. Further details about interface design can be found in the previous study [27]. Derived from the previous research [27], the warning window was defined as a 60\({}^{\circ}\)\(\times\) 30\({}^{\circ}\) rectangular area on the projection plane, where the center of this rectangle was located at the center of the pedestrian's sight. AR information was projected on the virtual projection plane in front of the eyes. The distance between the eyes and the projection plane was defined as the projected distance. According to the design of the HoloLens AR display, the optimal projected distance was 2 m, which minimized the discomfort from the vergence-accommodation conflict and ensured the optimal overlap of the two display lenses that a HoloLens has [28]. Based on the projection rules, the warning window size was proportionally adjusted to 2.31 m and 1.16 m from 2 m away. After the size and the position of the warning window were defined, the detailed components inside the warning window were designed. The vehicle location, the expected vehicle path, the expected conflict point, the collision time, the warning symbol, and the attention-leading component (i.e., a sidebar and an arrow to attract and lead the pedestrian's attention to the hazard) were included in the warning interface, derived from the previous study [27]. Figure 2 shows the layout of all these components presented in the warning window. Figure 3 shows an example of the user interface overlaid on the real-world view in which a vehicle was coming from the left and crossing straight in front of the pedestrian. Figure 3: User interface overlapped with a real-world view. Figure 2: Components in the warning window. As shown in Figure 3, an arrow sign was used to indicate that a vehicle was coming from the left side of the road. The location of the sidebar matched the location of the oncoming vehicle, and the arrow sign on the sidebar indicated that more information was available along the arrow direction. In this example, the sidebar with the left arrow on the left side of the warning window indicated that the upcoming vehicle was coming from the left side and could be seen by turning the head to the left. The conflicting time, 16 s, indicated that the vehicle was due to reach the red dot position in 16 s. The sidebar disappeared after the conflicting vehicle appeared in the warning window. The arrow position and the conflicting time updated in real-time based on the pedestrian heading direction.