Early speed camera designs made use of radar beams only. These have been shown to provide reading with high errors, sometimes ridiculous ones. In some countries, like Israel, this has resulted in a legal turmoil, where at some point (with the help of specialised legal firms) not a single offender was convicted in court. This has prompt a revision to the law and the calibration procedure of these cameras. But the problem was not quite solved.
To combat the issue, manufacturers have added a 2-frame camera (usually with 0.5s time interval) to the already existing radar system. The white lines on the road are used as mere rulers. Since the car will hide the ruler on the lane it is driving on, there's also a ruler on the adjacent lane. It just happens that if there's only one lane per direction, the adjacent lane will be on the opposite direction (but same direction if there're two lanes in each direction).
It seems that in the UK and Australia, a conviction is made if the error between the photo analysis and the radar reading is within a 10% margin.
You can read much more about this here.