Winding Road Sign
MUTCD W1-5
A series of curves is ahead; reduce speed.
What it looks like
A yellow diamond warning sign with a black arrow that snakes through several bends to show a series of curves ahead.
When you'll see it
You encounter it before a stretch of road with three or more successive curves, typical of mountain passes and country roads.
What to do
Reduce your speed before entering the section and stay alert through each curve, avoiding sudden steering or braking.
Related signs
Source: Manual on Uniform Traffic Control Devices (MUTCD), 11th Edition — FHWA.