Not long ago Saddle Road, which connects Hilo to Kona via the vast lava fields between Mauna Loa and Mauna Kea, was terra incognita for tourists, 60 miles of high-altitude pavement and potholes that were off-limits to rental cars. Technically it was two lanes, but the edges were patches of asphalt so working dudes and delivery vans would scream down the middle line intent on shaving a few minutes off the trans-island commute.
Now (completed in 2008) the first 40 miles or so from Hilo is wide and smooth, with shoulders and a lengthy passing lane coming uphill, and the last 20 miles is a real road with a double yellow line. Wider sections will open this year.
In the middle of Saddle Road is the accesses road to the celestial observatories on Mauna Kea, also a nice paved job, although above the Onizuka Visitors Center (around 9,000 feet) it remains 4WD. Thrill seekers can go the other way, south, and climb to above 11,000 feet to the weather station on Mauna Loa, navigating switchbacks and potholes through a Mars-like lava flow.