How does Doppler radar determine the speed of a vehicle?

Doppler radar determines the speed of a vehicle by analyzing the frequency shift of the reflected signal. When a radar signal is transmitted towards a moving vehicle, the frequency of the radar wave changes depending on whether the vehicle is moving towards or away from the radar source, a phenomenon known as the Doppler effect.

As the vehicle approaches, the frequency of the reflected signal is higher than the transmitted frequency, while a vehicle moving away causes the reflected frequency to be lower. The amount of change in frequency is directly proportional to the speed of the vehicle. By calculating this frequency shift, the radar device can accurately determine how fast the vehicle is traveling. This method is efficient and precise, making it a reliable technology for speed detection.

The other options do not align with how speed is measured by radar. Fuel consumption is unrelated to the speed measurement process, distance covered can determine speed in a basic sense but not specifically through radar technology, and color changes are not indicative of the speed of a vehicle. Thus, the focus on the frequency shift is what makes choice B the correct answer.