All radar is line of sight. It works by sending out a radio signal, the signal hits a target, and then is reflected back to the source. Early radar, WWII type, could detect an object and its direction but not its speed.
In the late 1800’s an Austrian physicist, Christian Doppler, discovered that frequencies change relative to an observer. For example, ... you know how that turbo ZX14 sounds as it approaches and then suddenly, as it passes you the pitch shifts? WWWHHHHEEEEEEE AAAAAWWWWWWHHHHHH (only a lot faster). It is called Doppler Shift, for the apparent shift in frequency.
(SIDE NOTE: Interestingly, in 1912 Vesto Slipher (an Indiana native) discovered red shift; that planets are moving away from us in the universe using Doppler Shift theory. The big difference is that Slipher applied it to light instead of sound. His discovery helped support the Big Bang theory.)
Radar became of age pretty much during WWII. It was used to show location and movement of objects in the sky (enemy airplanes) but could not calculate speed by the radar signal alone. Since then scientists have applied Doppler Shift to radar waves resulting, ultimately, in police radar that could detect a vehicles speed. Big Problemo! When the insurance companies found out about this they started buying radar guns for our local and state law enforcement agencies as fast as they could be made! (they are now supplying Laser guns which can target specific vehicles)
Say you are in a group of cars heading straight at the radar and one of them is going way faster than the others. Radar cannot tell which one is the fastest, only that something within the radar’s beam is moving fast. If several cars are doing 60 and one is doing 80 the radar will register 80. The LEO doesn’t know which one is doing 80 based on radar alone. He has to watch to see if one is moving faster relative to the other cars, or see a bike do a nose plant and all of a sudden the radar speed drops off. Through a combination of visually observing each vehicles speed in relation to the others and eliminating them one by one as they pass the radar the LEO gets his man.
Also, the radar beam widens as it moves away from the source. At a mile away it could be several hundred feet wide, covering 2 lanes of oncoming and 2 lanes of outgoing traffic. The radar will detect the highest speed but the LEO still has to visually identify the speeding vehicle. If there is only one then it's pretty easy. If there are many vehicles going both directions, unless the guy sees like Chuck Yeager, it would be hard to tell which one is speeding from a mile away, however they are trained to do this! The radar doesn’t select the specific vehicle, the LEO's eyes do.
Bottom line, radar doesn’t see over hills, around turns, through trucks, or through fat chicks on F1’s.