First of all, thanks for your answers.
Maybe I have misread what the book says. At first it talks about transitive and intransitive verbs in general, but at the end of the section it does give examples of transitive/intransitive pairs.
I had forgotten about using 一時間 to determine it, thanks for reminding me. In the case of 降りる for example, would it belong to group 2 or 3 depending on the meaning?
山を降りている action in progress (im descending the mountain)
バスを降りている result of a change (im off the bus)