Radar detectors.
I know they are illegal in Australia as the boys in blue dont like em much.

I have been reading some american web sites that have been touting them as almost mandatory .... apart from a few states such as Virginia ... apparently.
What i want to know is how do these work and how do the boys ... and girls in blue know you have em.
I cant understand how they would save your bacon.
I mean lets say you are doing a 130 kph down the Bruce Highway and round the corner you go and ...... whamo! you get gunned by the local constabulary.

i would have thought that the detector would warn you that you just got detected .... meaning its too late to slow down.
Obviously you can see that I am very brainy. I have not been able to put the right search criteria for a reasonable explanation.
your knowledge to asuage my curiosity is most welcome.