> When they require chains on 4wd, CalTrans (California Department of
> Transportation) usually closes the road.

Yes, they close the road for everyone that doesn't have 4WD and Chains.
This is for a specific level of chain control (I forget the actual name for
it) which dependent on conditions. It is not a total road closure.
If the conditions get worse, they may close it for everyone.

In some extremely rare instances,
> they will open for 4wd w/ chains required only when the snow is already on
> the ground and not still falling.

Well, it may be extremely rare for you down there in San Francisco, but it
is not all that rare up here in the Lake Tahoe area. Also, the 4WD & Chains
designation is not a total road closure and it's not limited to weather
where snow is not falling... I know from the inside :)

Bill