Helmet cams do not really give an accurate impression of the distance between bike and overtaking car. How do we know just how close a car has been when it overtakes, we have only a split second to judge the distance, and how good are we at accurately judging distance anyway? Strikes me, what we need is a distance sensor (cheap of course!) something like an echo sounder on a boat, but mounted horizontally. These echo sounders have adjustable alarms, so if the overtaker was less than the pre set distance, say one metre, then the alarm would buzz. Readings permanently recorded on graph paper sounds impractical, maybe there is another way. Over to you.