This is one of the toughest drives in the world. The road is steep and it's off road and a long way up! It's so high that you have to settle at base camp before attempting the off road portion of the drive. It's well worth the wait and the endurance as the views at the top are spectacular.
Mauna Kea on Hawaii island is the highest mountain in the world from it's base. The road to get to the access road is located between Kona and Hilo airports on Hawaii island.