In modern regulations, U.S. warning signs are classified as "Series W" signs, such as W1 Series (curves and turns), W2 Series (intersections), W22 Series (blasting), et cetera, ending with the W25 Series (concerning extended green traffic lights). Some U.S. warning signs are without category while others like the warning stripes at tunnel portals or plain red "End of Roadway" signs are classified as Object Markers (OM Series). In the U.S., "Stop" and speed limit signs fall under the R Series (Regulatory). Modern U.S. signs are widely standardized; unless they are antique holdovers from an earlier era, oddities like a yellow "Stop" sign or a red "Slippery When Wet" sign would typically appear only on private property—perhaps at a hospital campus or in a shopping mall parking lot.