How commute time per tile is calculated?

[OrSpeeder 72]

I am making a mod NOT related to transit...

 

But it DOES mod the bonus (or penalty) for desirability that is affected by commute time.

SC4 calculate the bonus by converting the actual commute time to a number from 0 to 255, then on vanilla it gives a -10 penalty for long commute, and 10 bonus for short commute, this shows in the interface when you query a residential tile (even one not developed yet!) it shows "Commute Time" and displays "Short, Medium, Long" as appropriate.

But I cannot figure how on earth it calculates that... I even made a city without neighbours to see what would happen, but had no success, in the neighbourless city, I managed to bring the citywide average commute time to 2 hours, and set NAM maximum to 10 hours (it is NAM default by the way, but I also played with other values), yet all residences show "long" commute...