Even though you would not think this would come into the forefront, it is actually a very interesting question. The answer, in fact, is one that you need to be aware of. Yes, police officers can use their radar guns while they are driving. Their guns can operate when both stationary and moving and when they are behind you or are in front of you.
This becomes important because it makes you realize that you are not safe even when they drive right past you. Your device should alert you of this occurrence and this is where range plays such a pivotal role. With a police car moving toward you, you will only have seconds to lower your speed. If the gun can be detected several miles away, it will work more in your favor.