They wouldn't be "invisible", they would be "fake" police. If they where not visible, it would defeat the purpose. You could do the same thing more cheaply by posting signs that say "speed monitored by automatic cameras." Then mount fake cameras on light poles or telephone poles ever few miles. A fake camera could just be a plastic box with a tube sticking out of it to resemble a lens. No need for power or anything. They could cost just a couple bucks.

I'm not actually advocating this however. I'm just saying how it could be done.

Years ago, but i can't remember where, a friend bought a $12 "radar detector trigger". You push the button, and it simulates a radar signal. It seemed to work the one time I tried playing with it.

Now, if you planted thoses once every mile, on a random timer, people would never know if it was a real radar or a false positive. That would help slow people down.

What I find helpful are those signs that monitor how fast you are going--same as the odometer, but I always assume a police officer is nearby.