What is indicated when a vehicle is detected moving away from the RADAR unit?

When a vehicle is detected moving away from the RADAR unit, it involves the Doppler effect, which pertains to the change in frequency of waves in relation to an observer moving relative to the source of the waves. As the vehicle moves away from the RADAR unit, the frequency of the reflected radar waves decreases, which indicates that the speed of the vehicle relative to the RADAR is a factor in calculating its actual speed.

The choice indicating relative speed change is correct because it reflects the difference in speed between the vehicle and the RADAR unit as the vehicle departs. The calculations involved can show whether the vehicle's speed is indeed increasing or decreasing relative to its last recorded speed when approaching the RADAR unit.

Other options, while related to speed detection concepts, don't accurately capture the specifics of when a vehicle is moving away. For instance, decreased speed could suggest a significant reduction of speed, while increased frequency would indicate a vehicle approaching, which is contrary to the scenario of moving away. Stationary position inaccurately suggests that the vehicle is not in motion at all, which contradicts the premise of detecting movement away from the unit.