Re Shing Mun Tunnel & Airport, other than it being the Old Road, I see this is only because SMT is only $5 and TSH is $8, and this usage pattern will most likely go away when $5 -> $8. (They say both side mileage is similar but TSH is friendlier on engine wear.)

Back to here, when I really think about it, I think you are right. If I am not mistaken from the AADT stats, it seems about 33% of relevant traffic actually goes through the Old Road.

To reiterate, I have no idea why the ratio is this high, and I really feel this should be `=tertiary`, but looking back at OSM principles, I should just take this as "the locals do be like that" and restore the Old Road to be `=secondary`.