I think people wrongly believe they interfere with speed detection devices. In fact it is the opposite the new laser speed guns need a nice shiny surface to reflect the beam. CD's in the windscreen are ideal!!
I'll tick the thread because I think its confirmed my suspicions.
Just as an aside, I used to sell radar and microwave equipment many yaers ago. Anyway, one of our customers were BAE (British Aerospace), so I had to visit one of their plants to inspect the installation of a waveguide joint. Afterwards over a cup of tea, the engineer asked if I had any information on RAM material, this is a material which is used to coat such things as water tanks to make them invisible to radar, so they wouldn't show up on the screen. Thinking I was going to make a sale he replied "Oh no, I only wanted to know the properties of the material my colleague has put around the front of his BMW to make it invisible to the police radar" it transpires he was always being stopped for speeding, needless to say this is illegal now.
I would loved to have seen the cops face as he zoomed passed at 90mph with nothing showing on their radar gun :-)