To be fair, this is my first time knowing about AADT, so I don't know how to respond to this.

It seems strange heavy vehicles are disallowed to go uphill but are allowed downhill.

From the many available bus videos passing by at the lower end, I can repeatedly see the signage to be something like this:
"Go forward for Sai Kung"
"Go left for Fei Ngo Shan"
No "alternative routes" specified for Sai Kung, which would otherwise justify `=secondary`. (I think Kam Tin Bypass has a "alternative route to Kam Tin" somewhere...)

It really seems like the Old Road is not supposed to handle cross-district traffic. It actually being physically capable of doing so seems like a coincidence, so it really cannot be `=secondary`.

Again, this feels like a local road that is extra long and open on both ends.

How do you think about these?