Mythbusters is a bunch of BS. They don't test the science well enough to bust anything. They just do enough to make an interesting TV show.
Now, on to the specter guard. The tech is sound. Have you ever seen a DVD with a "hologram" cover, where the picture changes based on the angle you are holding it at? This is the tech being used here. The argument is that these speed cameras have a specific angle they have to use in order to get an accurate read on your speed and a pic of your plate. In theory one could make a cover that obscures your numbers at those angles.
Should you buy it? No. For it to be effective it would have to obvious to the naked eye you are using it. Your plate would look obscured to every angle excepts straight on. No one gets down on their hands and knees to look at your plate. Also they are illegal (since you can't have anything cover your plate) so get caught with it on your car can get you even more tickets. If you are just looking to avoid speeding tickets you be better off stealing some one else's plate. At least thats not as noticeably illegal to the naked eye.
Note: I do not condone or suggest you steel a license plate.
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