  The use of reactive detection technologies such as passive and active sensors
for avoiding car accidents involving pedestrians and other Vulnerable Road
Users (VRU) is one of the cornerstones of Cooperative, Connected, and Automated
Mobility (CCAM). However, CCAM systems are not yet present in all roads at all
times. The use of currently available technologies that are embedded in
smartphones, such as location services and Internet access, are enablers for
the early detection of VRUs. This paper presents the proof-of-concept of a
system that provides vehicles with enough information about the presence of
VRUs by using public cellular networks, an MQTT broker, and IEEE
802.11p-enabled hardware (a roadside unit and an on-board unit). The system was
tested in an urban environment and in a test track, where its feasibility was
evaluated. Results were satisfactory, proving the system is reliable enough to
alert of the sudden appearance of a VRU in time for the driver to react.
