It is the perfect sheeting material for yellow warning signs such as a Railroad Crossing, Stop Ahead, or Chevrons.

"I have not moved for five hours," said Zhang Xingping, 27, standing outside his cab near a road traffic sign mockingly warning him to obey the 100km per hour speed limit.

Street signs in the United States follow a simple pattern of shapes and colors: Yellow triangles are cautionary, red octagons are imperatives, and green rectangles are informative.