No Left Turn Sign
MUTCD R3-2
Left turns are prohibited at this location.
What it looks like
A white square with a black left-turn arrow inside a red circle crossed by a red slash, indicating the movement is prohibited.
When you'll see it
Posted at intersections and driveways where a left turn would be unsafe or would conflict with heavy oncoming or cross traffic.
What to do
You must not turn left at this location; continue straight or choose a permitted movement instead.
Related signs
Source: Manual on Uniform Traffic Control Devices (MUTCD), 11th Edition — FHWA.