What do you mean radar is not a problem, laser is.
There are Laser radar and K/Ka band radars.
The reason why K/Ka band radars is easy to detect is that beam from that radar is wide and covers 4-5lanes. So when cop radaring with K/Ka radar he doesn't really know whose speed radar shows. He just pulls over the fastest driving car that he sees.
Why Laser radars are bad? Well laser radar has to be pointed to the car and on to the flat surface. That's why laser radar has optical aim, cause beam of the radar is very small it cops have to aim it to every car.
So when you have radar detector in your car, it detect K/Ka radars cause beam of that radar is spreader out all over the road, so when cop radaring a car in front of you, radar detector can catch it. And since laser beam is so small your radar detector will only show you that there is a cop when a beam will be pointed at you. When means, too late buddy, get a ticket.
P.S. i'm not really sure how non-laser radars called, but i think they are K/Ka band