I was travelling along the motorway this morning and saw one of those mobile speed cameras fitted into a van parked on a bridge above the opposite carriageway. I got to wondering how this type of camera judges the speed at which a particular vehicle is travelling when there are no calibration marks painted onto the carriageway, as there are for fixed cameras. Also how does it differentiate between traffic travelling at different speeds in adjacent lanes? My own personal opinion is that they allocate a random speed in excess of the speed limit to each snap and send out the fine regardless, banking on people paying the fixed penalty because they don't want the hassle of a court appearance! Does anyone know the answer, or have any decent conspiracy theories?