You probably already know the answer to this question but believe it or not, it is one that gets brought up on occasions. Logically, it is a question that makes sense to ask as the purpose of radar detectors is to lessen the likelihood you get speeding tickets. However, and you know this, they will not make you immune to them. As mentioned earlier, this is one of the reasons why some locations prohibit the technology. With the knowledge that you can detect when a radar gun is in the area, it can promote reckless driving.
But, police officers have been at this game for a long time. If you are going fast enough, a radar gun will not be needed to tell that you are speeding and remember, these devices will not simply detect oncoming police cars.