Once again I would say, if you want to waste $300, go for it. Just because I am a Police Officer now doesn't mean that I always have been. It also means that I used to and still do, love to go fast. The only difference between you and me is that now I get to see the "other side". The funny thing is that so many of the detectors now talk about being able to pick up the laser, instant on, "computers" (haha), and a few other things.
The supposed "computer" that they MAY be able to pick up if you are lucky would be the instant on type radar that like someone said, is braodcasting some small amount of signal all of the time. The ironic part is that most Departments will still not spen huge amounts of money on the higher dollar radars because there is no need to. Most still buy the K or KA band radars. Where you are getting fooled is that by them saying that they can just pick you up by the computer, it gives you the false sense of security.
You would not believe the number of tickets that are given to people with radar detectors. About the ONLY way that they do you any good is for you to treat EVERY single signal or warning that you get as a Police radar and no matter how much the strength of the warning. As for how many Officers that will keep the radar on all the time. I know of one out of 200 at my department who is just to lazy to use it correctly. Out of those 200, I would definitely say that if you are caught speeding and a detector is seen, you would get the ticket.
Detectors are pretty cool right now because of the "weather and traffic" warnings that they broadcast and that's about it. Also, don't be lulled into that whole thing that K40 says about, "We'll pay for your ticket if you get one". They will pay your fine which unless you were FLYING is only around $10-15. Even if you were flying, it may be $25. Then you have court costs and insurance points to follow.
The best advice that anyone gave in this whole post is to use your eyes!! Unless you run into the Officer that is running the radar ALL the time, they will have to see you and flip the switch to clock you. All you have to do is hit the brakes to slow down quick. Most Officers that I know,do not feel comfortable about going into court if they weren't able to lock the speed in, which is another step that gives you time to slow down.
Like I said, your money, but there are plenty of other things to spend $300 on rather than to waste it on a detector.
