> How often are bolders in the road?

as someone who has driven all over the west , all the time.

You have New Mexico and Arizona that believe that steep incline cliff faces on both sides of the highways are a fashion statement, states like California, Washington, and Oregon that like to build new properties on rapid-eroding mountainsides, and states like Nevada that simply don't patrol sparsely populated highways -- and are simply ignorant to the boulder some kids rolled into the street some weeks back.

All anecdotal, but i'm a fairly heavy driver and I see them commonly.

I agree that false positives are to be avoided -- but that doesn't really lessen the value in boulder-avoidance ; it just brings up yet another point to worry about with the system as a whole.