Legally, motorway regulations begin when one passes a start of motorway sign (bridge over dual carriageway, colloquially ‘chopsticks’) and end when one passes an end of motorway sign (bridge over dual carriageway, struck through with a red diagonal line). AFAIK, motorways are mapped on OSM as starting at the point of divergence from a lesser road and the point of convergence with a lesser road. A few rare exceptions occur where the start or end motorway sign is set back significantly from the point of convergence or divergence, e.g. junction 30 of the M25. I believe the same procedure is applied to the start or end of lesser roads like trunk roads, primary roads and secondary roads. Since the start or end of lesser roads aren’t as clearly signed as motorways, OSM maps them like motorways. In Scotland, trunk roads function similarly to motorways and have a defined legal point where they start and where they end. However, this is not represented by signage on the ground because they have no regulations which prohibit certain modes of transport like motorways. Personally, I disagree with the rare exceptions to the rule laid out above for the sake of easier navigation, but I believe there’s consensus for them to be mapped that way. However, with the trunk and primary road changes I outlined above, there’s no consensus for this and editors like myself have been reverted before for changing the classification of highways around Edinburgh. I believe there should be discussion before further changes are made to the trunk and primary roads in question.
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