Why do you think the sensor can be used to detect the speed of a moving object? Perhaps it detects changing interference patterns, rather than a Doppler shift.
In contrast to speed guns, which use a wave guide to direct the antenna radiation pattern, the X-Band Motion Detector's antenna has a wide radiation pattern to convert velocities from multiple points to oscillations that notify the microcontroller that movement was detected. This device is designed to detect motion, not to determine speed of a moving object. However, an application may still use a rearranged version of the Fd equation to determine speed provided everything else in the detection area stays still.