# An Augmented Warning System for Pedestrians: User Interface Design and Algorithm Development
## 1 Introduction  
Compared to vehicle drivers, other road users such as pedestrians or cyclists are more vulnerable when sharing the road with vehicles. Each year, 1.35 million fatalities and 20-50 million injuries worldwide are reported due to traffic accidents [1], and the overall number increases yearly [2]. Among these injuries and deaths, the rate of pedestrian deaths remains high and keeps rising [3], which can be linked to many moderating factors. For example, pedestrian injuries due to phone distraction increased more than three times from 2007 to 2010 [4]. Further, decreasing situational awareness could also lead to a pedestrian engaging in risky behavior during road sharing [5]. Due to the limited sensor accuracy and reliability, the rapid implementation of autonomous vehicles may lead to even higher pedestrian casualties [6]. It is necessary to develop a solution to reduce the potential threats from these known factors, especially when the pedestrian's view is blocked by a truck, bus, or building or when the pedestrian's situation awareness is low due to fatigue, distraction, etc. Due to such visual blockages, it could be very beneficial to have a visual-based warning system to make up for the information missed by the pedestrians. As a warning system, the information must be carried out in real-time. Moreover, due to the limitation of pedestrian's capacity, the warning system must be light in weight and easy to carry while the pedestrian is walking in public. Different methods were developed to improve pedestrian safety and warn road users, though most of them focused on detecting the collision rather than warning the pedestrianof the potential risks [7]. Limited methods were developed to carry out warning pedestrians when a potential collision was detected. Smartphone audio [8] and vibration [9] were used to provide warning information to pedestrian users, however, they could not provide the multimodal cues as a graphically based information system can [10]. [PERSON] [8] proposed a phone-based visual warning system for distracted pedestrians at intersections. Such a system, however, requires real-time interaction between the pedestrian and the mobile phone to deliver the warning message. A visual system could present the warning information more directly and comprehensively compared with auditory- and tactile-based systems, which made it the best media to deliver an immediate warning to the pedestrians while giving them more time to respond [11]. However, pedestrians have limited carrying capability during walking, which limits the types of visual systems they can carry. It is also not safe to forcibly shift the pedestrian's attention from surrounding traffic to the visual system while sharing the road with other road users. Due to these limitations of pedestrians, traditional visual-based warning system designs using a large screen or a mobile phone are not feasible for pedestrians. Augmented reality (AR) is a state-of-the-art technology that can \"augment\" the real world with virtual information so that users can perceive the augmented virtual information without losing their sense of the surrounding environment. Current devices that can display AR information include eyeglasses [12], heads-up displays (HUD) [13], contact lenses [14], handheld devices (e.g., smartphones) [15], etc. Since some of these AR devices could be carried with minimal effort (e.g., eyeglasses), they could be used as a feasible choice to present a visual warning system for pedestrians. AR technologies are applied in the design of vehicle-based warning systems. [PERSON] [16] developed an in-vehicle HUD user interface for crash warning systems. [PERSON] [17] evaluated different AR warning systems for the driver in an urban environment. Among these existing studies of warning system design, the research mainly focused on warning drivers rather than pedestrians. In addition, most of these existing studies rely on a mounted HUD rather than on a wearable display. Such mounted displays could not be transferred to pedestrian application.