Signal Ahead Sign
MUTCD W3-3
A traffic signal is ahead; be prepared to stop.
What it looks like
A yellow diamond warning sign showing a black symbol of a traffic signal with red, yellow, and green lights to warn of a signal ahead.
When you'll see it
It appears before a traffic light, especially where the signal is hard to see because of a curve, hill, or high approach speed.
What to do
Slow down and be ready to stop, since the signal ahead may be red or about to change.
Related signs
Source: Manual on Uniform Traffic Control Devices (MUTCD), 11th Edition — FHWA.