K40 radar and lidar sucks in test that I have witnessed first hand.
It depends on where live if rear protection is need. In FL and GA I have read they do use rear shots.
I have a POS K40 radar detector that came with my car. My V1 CRUSHES it by 3-4 seconds every time. I even ran the K40 in Hwy mode and my V1 in big L mode. Guess who wins every time? V1.
Try to find a Duracell 4A battery for the remote for the K40 system. Only Duracell will work because the end are different on the other brands. K40 know these and they will send you a new battery free of charge. Yes, this is a "AAAA" battery for those you who didn't know this.
K40 made good CB antennas back in the day. They should have stuck to what they now.
Plus there is a new LIDAR gun out the K40 won't jam. Ecsort shifters have simlar issues to K40 but they do jam a little better when the jam.

 

They switched from AAAA batteries to AAA some time ago. The K40 in my wife's car had the AAAA batteries, and they were hard to find (only some drug stores carry them). I sent the remote plus a check for $10 to K40 and they sent me a new remote that uses AAA batteries.

Valentine makes a very nice product, but I've never wanted to deal with leaving a radar detector on the dash, or having to put it away each time I park. If I had to do that, I would only use it some of the time, and most likely get a ticket when I hadn't bothered to plug it in.