Which method allows for real-time speed measurement of a vehicle?

The method that allows for real-time speed measurement of a vehicle is radar. Radar operates by emitting radio waves that reflect off a moving vehicle. The radar unit then calculates the speed based on the Doppler effect, which is the change in frequency or wavelength of the waves in relation to the speed of the vehicle moving towards or away from the radar source. This technology provides instantaneous speed readings, making it a reliable method for law enforcement to monitor vehicle speeds in real-time.

Pacing, on the other hand, involves an officer following a vehicle at a constant speed to determine its speed relative to their own vehicle. This method can be somewhat less immediate, as it relies on the officer's ability to match speed and measure over a distance. VASCAR, which stands for Vehicle Speed Computer and Recorder, uses a combination of timing methods and distance markers to calculate speed, but it requires manual input and is not as instantaneous as radar.

Thus, radar stands out for its ability to deliver immediate speed measurements without the need for additional calculations or lag time associated with other methods.