As far as Northern Maui goes, you are correct that the roads become very thin. However, there is at least one road that circles the island. The lane is narrow, but the road is public.
There are many private roads on the islands because the road is owned by one of the major land holders. They are private roads, usually traced to the history of sugar cane growing in the area.
Regarding the lengthy drives on Maui, the need for infrastructure investment is clear. However, productivity on Maui is somewhat limited by the lack of economic infrastructure. The democrats have failed to create a climate conducive to an economic expansion due to higher taxes and poor exportation of goods (which limits wealth accumulation).
Highway by-passes would be useful, but the economic impact of such development is limited by the lack of cooperative politics.