## 4 Intelligent Wearable Solutions for VRU Safety: Trends, Challenges, and Opportunities
### Wearable Technologies for Two-Wheeler Users  
Research on enhancing traffic safety for two-wheeler users through wearable technology encompasses 16 studies (Table 7). These studies address various types of two-wheeler users, including cyclists [41; 42; 43; 44; 45; 46; 47; 48; 49; 50], scooter riders [51; 52; 53], and motorcyclists [54; 55; 56]. Existing research on wearable devices for two-wheeler users can be primarily categorized as smart helmets [41; 44; 45; 46; 47; 51; 54; 55; 56], smart glasses and augmented reality displays [49; 50; 53], smart clothing, such as jerseys and vests [43; 48; 50], wearable sensor systems [42], and specialized footwear [52]. With advancements in connected vehicle technology, research on cyclist safety systems has evolved from single-function to multi-function integration and from unimodal to multimodal interaction approaches. [PERSON] et al. discovered that unimodal signals are suitable for directional cues, while multimodal signals are more effective for emergency situations, successfully reducing children's reaction times and accident rates [41]. [PERSON] et al. developed a helmet-mounted display system that warns against \"dooring accidents\" [44; 46], proving that audiovisual cue combinations enhance cycling safety, with participants strongly preferring interactions incorporating auditory feedback [47]. Their subsequent research [49] emphasized that effective warning systems should be concise, intuitive, and display only critical hazard information. The systematic evaluation [50] of [PERSON] et al. further validated this direction, finding that arrow visual cues were most efficient in unimodal interfaces, while visual-haptic combined feedback significantly improved user experience while reducing cognitive load. To achieve these objectives, researchers have explored technical solutions for improving bicycle safety through two key dimensions: perception and interaction. In perception technology, [PERSON] et al. [42] and [PERSON] et al. [43] focused on rider behavior recognition and physiological monitoring, respectively--the former achieving high-precision detection of starting movements through smart devices, while the latter developed an integrated solution combining physiological monitoring with light signaling systems. For interaction methods, the haptic feedback helmet [45] of [PERSON] et al. demonstrated that head-based tactile feedback efficiently communicates directional and proximity information, while the smart vest of [PERSON] et al. [48] showcased the value of adaptive visual feedback in low-visibility conditions. These studies collectively indicate that cycling safety systems must flexibly select information modalities based on specific contexts--maintaining simplicity in ordinary situations while appropriately increasing multimodal redundancy in high-risk scenarios--while carefully avoiding information overload to ensure balanced attention allocation for cyclists. Overall, cycling safety research demonstrates a progression from purely technical validation toward user experience optimization, with research priorities