About £100m of transport costs that take place in Scotland are not allocated to Scotland in GERS
The first observation worth making when it comes to transport cost allocations in GERS is that none of these relate to infrastructure projects in the rest of the UK. The most commonly (incorrectly) cited example is High Speed 2 (HS2) - there are no HS2 costs allocated to Scotland in GERS.
Perhaps more surprisingly, not all transport costs that take place in Scotland are allocated to Scotland in GERS. About 30% of Scotland's ferry costs (~£100m) are allocated to the rest of the UK in GERS. The logic applied is HM Treasury's: they assume this is the proportion of value delivered to the rest of the UK "based on 2023 Visit Scotland Island Visitor Survey. This is an occassional [sic] survey caried [sic] out infrequently".