There are several different kinds of asbestos and they differ radically in their level of risk and the worst kinds are not (generally?) found in California. So that signage might just be out of an abundance of caution.

I wouldn't be surprised if by the numbers you have a lot more risk of serious injury driving to and from the hike than from the asbestos, particularly if your hiking doesn't involve intentionally disturbing the ground. :)

For the many posters on HN in the bay area-- watch road cuts for green rocks, asbestos co-occurs with serpentinite all over the coast range-- and serpentinite is the state rock! You can easily find some with asbestos, usually grey/white fibrous strands on the rocks. Don't grind it up and huff it, but you can thrill your friends by showing them some boogieman-mineral you found.

(It's only fitting that the state rock is technically known to the state of california to cause cancer... :P)

> that signage must just be out of an abundance of caution.

I'm pretty sure prop 65 applies to all materials known to man, so that checks out

Prop 65 requires me to tell you that the device you are reading this on contains chemicals known to the state of California to cause Thinking-regulations-are-stupid in mice.