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In August 2000, Mike Ballard reported that Caltrans had corrected the sign. 
State route 152 runs from CA 1 in Watsonville to CA 99 in Chowchilla, and part of it is a controlled-access expressway. Even if it were promoted to interstate status, keeping the number 152 would be very unlikely. After all, California hasn't used up all its x38's yet.
See also: Full-size photo.
Signing errors are a popular topic on misc.transport.road, and California is not alone. Connecticut, for example, updated a big green sign for US 5 and CT 15 with the markers switched, giving state route 5 and US 15.