Speed camera vans utilise the K Band frequency –
Unfortunately the speed camera and radar inside the van are usually focussed pointing across to the opposite side of the road, not along the road in the direction of traffic flow. This means the radar detector has to be nearly beside the speed camera van before it picks up the signal.
The radar systems in patrol cars are pointing in the direction of oncoming traffic (or traffic following behind). This is much easier to detect compared to a mobile speed camera set up inside a van on the side of the road, simply because the technology is totally different.