Slower Traffic Keep Right Sign
MUTCD R4-3
Slower-moving vehicles must keep to the right lane so that faster traffic can pass on the left.
What it looks like
A vertical white rectangle with black legend reading SLOWER TRAFFIC KEEP RIGHT in bold capital letters.
When you'll see it
Posted along multi-lane highways and grades where slower vehicles tend to hold up faster-moving traffic in the left lane.
What to do
If you are moving slower than surrounding traffic you must keep to the right lane, leaving the left lane clear for passing.
Related signs
Source: Manual on Uniform Traffic Control Devices (MUTCD), 11th Edition — FHWA.