What is a common characteristic of radar speed devices?

Radar speed devices function by utilizing the principles of radar technology, specifically the Doppler effect, to measure the speed of moving objects. When a radar device emits a radio wave towards a vehicle, the wave reflects off the vehicle and returns to the radar unit. The frequency shift between the emitted and reflected wave indicates the speed of the vehicle.

This mechanism is why the option regarding measuring speed based on reflective signals is accurate. The device effectively relies on these signals to determine how fast an object is moving, making this characteristic fundamental to the operation of radar speed detection devices.

The other options lack relevance to how radar devices function. For instance, while radar can detect multiple types of moving objects, it is not specialized for all types, and it does not require direct visual contact with vehicles, as it can measure speed from a distance based on reflected signals. The mention of operating in a vacuum is also misleading as radar devices function in the atmosphere, where they can still achieve accurate readings.