can a police officer scan my speed, while in motion in the same direction

in front of me? is it possible for a police officer to record my speed as i am approaching him, as he is in motion on a highway going the same direction as me? Another words, we are both heading the same direction, my speed is greater than his, he is in front of me, and I am approaching him.. thanks, and sorry if it is a bit confusing.. Also if it makes a difference, im in the state of Illinois.
 
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