The higher the R factor, the more common sense you need when visiting a place. Most UK sites have a relatively low R factor, with plenty of railings and other safety equipment. Lanzarote's R factor is much higher, with some railings, but seemingly designed to pitch you over cliffs with their knee-high bars. The same is true of New Zealand, where the higher the value of R, the more fun Kiwis have. Much of the US has a low value of R, but the average is pushed up by places like Arizona's Horseshoe Bend and, well, most of Big Island.
There's nothing to really stop you walking into the maw of an erupting volcano - just a few signs, and your common sense. After all, lava is hot, and it can be quite damaging to one's footwear...
Somewhere under the lava flow behind this sign is the village of Kalapana.