Speed trap causing accident

Colleague of mine just got into the office having witnessed a crash caused by a speed trap. A90 into Edinburgh, on city boundary, dual carriageway going from 60 to 40. There''s a slip lane onto the carriageway from the left, which is controlled by a ''feeder'' traffic light with a camera. Police were there with a mobile laser trap this morning at the same place as the light-jumping camera, and a Saab came past (meant to be doing 40) and hit the brakes on seeing the trap, and following white van went smack into the back of it. OK, participants might not have been exercising due care and attention, between speeding and tailgating, but without the speed trap there would not have been an accident... Neil