Several years ago I investigated the Average Speed Camera information, as they are mostly all the way from Inverness down to and beyond Perth (on the A9). Even though you may see an average speed camera at the start and end of each short dual carriageway stretch, they don't actually cover speed data along all the dual carriageway sections, only along the single roads. As such this means you can speed up to overtake many vehicles every time you go along those dual carriageway bits.