HQ have released some information about when routing will avoid a 'detour' from a major road.
Here's a paraphrase of what they said, as I understand it, of how to prevent detours from happening:
- Detours go between two segments with the same name, and with the same road type via a different road type.
- Freeways and Major Highways: Detours are avoided if they are less than 5km.
- Minor Highways: Detours are avoided if they are less than 500m.
This means, I think, that a) our hacks for keep left/right are probably breaking this mechanism,
and b) large junctions on minor highways may have detours longer than 500m. A typical GSJ has at least 600m of ramps, not counting the roundabout.