Traffic radar determines the ____________ of a target vehicle by measuring changes in the frequency between the point that the signal leaves the antenna and returns after being reflected by an object.

Traffic radar determines the speed of a target vehicle through the Doppler effect, which involves measuring changes in the frequency of the radar signal as it bounces off the moving vehicle. When the radar signal is transmitted, it encounters the vehicle and reflects back to the radar unit. If the vehicle is moving towards the radar, the frequency of the returned signal will be higher than the transmitted signal, whereas if the vehicle is moving away, the frequency will be lower.

This frequency change is directly related to the vehicle's speed. The radar system uses this frequency difference to calculate how fast the vehicle is traveling. The relationship between the frequency shift and the speed of the vehicle is well defined in physics, allowing for accurate and reliable speed measurements under various conditions.

While distance, direction, and acceleration are important concepts in the context of vehicle motion, they do not directly relate to the function of traffic radar in measuring how fast a vehicle is moving at any given moment. Thus, the primary function of traffic radar is to ascertain the speed of the target vehicle using the principles of wave frequency changes.