What it looks like
A horizontal red rectangle with the words WRONG WAY in large white capital letters.
When you'll see it
Posted beyond a Do Not Enter sign on freeway ramps and one-way streets, catching drivers who have already begun heading against traffic.
What to do
You are traveling against traffic and must stop, safely turn around, and return to the correct direction immediately.
Source: Manual on Uniform Traffic Control Devices (MUTCD), 11th Edition — FHWA.
