It’s a total crock. There’s only four ways to defeat radar sensing:
(1) Early detection (via radar detectors)
(2) Absorption (don’t reflect the radar)
(3) Controlled reflection (reflect the radar away from the source)
(4) Jamming (return a false signal to the source)
#2 requires expensive materials and even then has limited effectiveness. #3 sounds a lot easier than it is because it’s very difficult to build an object that doesn’t have some edge or curve that can return a reflection.
A CD hanging from the mirror is not going to affect the absorption or reflectivity of the rest of the vehicle. This is because radar receivers expect some level of noise and bypass. They use an averaging technique and your little CD just doesn’t figure into the average in any significant way.
So this leaves only jamming as a possibility. To jam the radar sensing equipment, you need to send an amplified, red shifted radar signal back to the source. Therefore, jamming is an active thing and CDs are passive things. They have no facility to alter the reflected radar signal back to the source.
Now, I know what some of you are thinking… “Wait a second. The laser radar could hit my dangling CD and reflect back an altered signal. After all, we’ve all seen the prismatic effect of light reflecting off of CDs.” Unfortunately, it doesn’t work that way. The laser versions use an encoding technique called “spread spectrum”. It’s actually similar to the encoding used in cellular and cordless phones to insure others can’t easily listen in on your conversations. One of the reasons that lasers were added to police speed sensing equipment is that they can’t be jammed. To jam the laser system you would need to send a properly encoded signal to the receiver BEFORE you received the original. Einstein tells us that this can’t be done… I tend to believe him.