robbo is correct. Clarkson was bemoaning the dot matrix signs on the M25 that say congestion ahead, slow down to 50.... and there never is any. It's because, as robbo says, the cameras only come on in conjunction with the reduce speed warnings. if the traffic is flowing normally (without warning signs of conjestion) it doesn't matter how fast you are going, the cameras won't flash. They were also standing in front of a camera later in the show waiting for someone to speed through it and trigger the flash so they could have a "portrait" taken of themselves.