The marker reads:

In the first quarter of the 20th century, as the automobile gained in popularity, a system of roads began to develop informally through the actions of private interests, these were known as auto trails. They existed without the support or coordination of the federal government.

One such roadway was the Jefferson Davis Memorial Highway, which was sponsored by the United Daughters of the Confederacy. The road started in Washington DC and traveled through the southern states to its terminus in San Diego.

More than ten years after the construction of the Jefferson Davis was begun, it was announced that the highway would be extended north out of San Diego and go the Canadian border as Highway 99.

Old Highway 99 in northern Siskiyou County is now Interstate 5.

Source: Wikipedia