Implementations a method using a collision detection device associated with a user to detect a moving object in an environment and provide an alert to the user are provided. In some implementations the environment comprises at least one transmitter of opportunity. In some implementations, the collision detection device comprises a receiver and a processor. In some implementations, the method comprises receiving at the collision detection device Wi-Fi signals reflected from the moving object where the Wi-Fi signals originate from a Wi-Fi source not associated with the moving object. The method further comprises detecting, measuring, and tracking the Doppler effect of the Wi-Fi signals at the collision detection device to track velocity vector relative to the collision detection device. The method further comprises calculating the time of arrival of the moving object based on the velocity vector relative to the location of the collision detection device. The method further comprises tracking the relative angle between the moving object and the collision detection device based on the velocity vector. The method further comprises predicting the occurrence of a collision between the moving object and the collision detection device based on the relative angle. The method further comprises providing a notification based on the predicting step.