In this paper, a dedicated on-board ‘sensor’ utilizing wireless communication devices was developed for the collision avoidance around intersection. Four wireless receivers of Zigbee are installed in the four corners of the driver's vehicle. Each receiver is shielded and has slightly directivity. These effects enable ‘sensor’ to estimate positions of transmitters, which is brought by pedestrians, bicycles, motor cycles, and vehicles based on comparison of four signal strength of four receivers. Positions are obtained relative to the driver's vehicle as combinations of four directions, near or far, and approaching or leaving. Since the estimation algorithm is focusing on relative values among four wireless receivers, the detection results should not be affected by the transmission power. On-board sensors utilizing vision, LIDAR and radar cannot detect objects hidden by road facilities and other vehicle. Although infrastructure sensors for Vehicle-to-Infrastructure cooperative systems can detect such the hidden objects, they are much expensive than on-board sensors. The on-board wireless ‘sensor’ developed in this paper would be an alternative method for the collision avoidance around intersections.