I get there there are few, but why are there any at all?
Overly the years I've traveled Route 1 and other incredibly windy and narrow roads in Northern California that abut cliffs with terrifying drops. Periodically you see a guard rail. Sometimes the guard rail is alongside a giant cliff, but just as likely it is next to a small gully or sometimes just a field. Usually there is no guard rail at all.
I don't understand what determines whether to put a guard rail in - is it a 90 degree drop? A neighbor complaining? A response to someone flying off the cliff?
I'm less interested in why there are no guard rails, and more interested in why the ones that are put in are there, since there are so few and seem(?) to be strategically placed.