California is marking the historic route of US 99, once the major north-south route through the state, and now replaced in upper northern California by Interstate 5.
In the rest of northern and central California, US 99 was simply replaced by California 99, but it ends at Interstate 5 in Red Bluff. At Red Bluff (left), the Historic US 99 designation continues downtown along Business Loop 5.
Farther north, in Yreka (right), California 3 and California 263 comprise Historic US 99.
Also note that the signs attempt to simulate the button reflectors often used on green guide signs in California. Those reflectors are no longer available and are now being replaced by reflectorized sheeting, but California still pays homage to the button reflectors on these signs.
It seems to me that the fonts of the Red Bluff sign are more historically accurate (as it were) than those of the Yreka sign.
These photos were taken in September 2004.