Some rental cars still prohibit you using that road but they've really improved it in recent years. Used to be a very dangerous road but not so bad now.
The new extension to 180 bypassed the dangerous part and it all flies now. Just watch for the cops descending into Hilo with speed limit drops going downhill.
The road you describe is, I think, the one where more than one bicyclist has died when they failed to make the corner and flew into the volcanic rocks.