Takes about 4 hours or so. The southern half to Kona is the fastest. Avoid Kona at 4-5:30pm. It is rush hour, and if going south, lots of traffic.

Saddle road is not any faster than going around.

When the Rental car company states "do not go on ....., and you will have no insurance on ....", think about what that means. You hit somebody and injure them, they own you, unless your personal car insurance somehow conveys.