Caltrans has permanent CFSs on I-80 over the Sierra to issue winter
advisories or closures, but I have not seen them change a speed limit ad
hoc. Caltrans does have slower speed limits for chains - 35 mph if I
remember correctly. I do not know of any urban winter restrictions in
California. Most of our urbs would come to a complete standstill in snow (as
happened in the East Bay back in 1973).
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