For the second time this year, someone has stolen road signs along Nicasio Valley Road. The Department of Public Works reported the theft of a dozen signs to the Marin County Sheriff’s Office on Friday. They include signs urging motorists to proceed with caution in a school zone and warning them to keep alert for horses and deer crossing the road. Replacing the signs will cost roughly $5,400, according to Sgt. Adam Schermerhorn, a sheriff’s office spokesman.  Earlier this year, eight traffic signs were reported stolen or damaged in the same area, he said.