As smartphones gain their popularity, vulnerable road users (VRUs) are increasingly distracted by activities with their devices such as listening to music, watching videos, texting or making calls while walking or bicycling on the road. In spite of the development of various high-tech Car-to-Car (C2C) and Car-to-Infrastructure (C2I) communications for enhancing the traffic safety, protecting such VRUs from vehicles still relies heavily on traditional sound warning methods. Furthermore, as smartphones continue to become highly ubiquitous, VRUs are increasingly oblivious to safety related warning sounds. A traffic accident study shows the number of headphone-wearing VRUs involved in roadside accidents has increased by 300% in the last 10 years. Although recently a few Car2Pedestrian-communication methods have been proposed by various car manufacturers, their practical usage is limited, as they mostly require special communication devices to cope with the wide range of mobility, and also assume VRUs' active attention to the communication while walking. We propose a smartphone-based Car2X-communication system, named WiFi-Honk, which can alert the potential collisions to both VRUs and vehicles in order to especially protect the distracted VRUs. WiFi-Honk provides a practical safety means for the distracted VRUs without requiring any special device using WiFi of smartphone. WiFi-Honk removes the WiFi association overhead using the beacon stuffed WiFi communication with the geographic location, speed, and direction information of the smartphone replacing its SSID while operating in WiFi Direct/Hotspot mode, and also provides an efficient collision estimation algorithm to issue appropriate warnings. Our experimental and simulation studies validate that WiFi-Honk can successfully alert VRUs within a sufficient reaction time frame, even in high mobility environments.