What does a yellow diamond sign with a wavy arrow mean?

A wavy or wiggly arrow (like a car skidding) on a yellow diamond is the 'slippery when wet' warning. It indicates a section of road where the surface becomes especially slick in rain or other wet conditions — typically because of pavement type, smoothing from heavy traffic wear, or grade. Reduce speed in wet weather where you see this sign, avoid sudden steering or braking, and increase following distance. Slippery-when-wet warnings often appear on bridges, in areas of frequent oil drippings (intersections, freight terminals), and on smooth asphalt where polishing has reduced surface texture. These are also areas where hydroplaning is more likely at speed.