Slower traffic should use the right-hand lane. The sign is used on roads with several lanes in the same direction.
In the United States, R4-3 is a vertical white rectangle with a black border. Its black lettering reads SLOWER TRAFFIC KEEP RIGHT.
Text signs in the United States and Canada
Slower traffic keep right
Keep to the right-hand lane if travelling more slowly than the normal flow of traffic.
How it looks in each country
Official sign in each country; the code is its number in the national rules. Signs that come in two directions show the left and the right version.
ChileCode RO-2a 
PhilippinesCode S2-7 
TaiwanCode 指27 United StatesCode R4-3
Related signs
Image credits
Sign images from Wikimedia Commons, used under their licences.
- Chile RO-2a: Government of Chile – National Traffic Safety Commission, Public domain
- Philippines S2-7: Department of Public Works and Highways (Philippines) Sign redrawn by TagaSanPedroAko., Public domain
- Taiwan 指27: 中華民國交通部, Public domain
- United States R4-3: Federal Highway Administration – United States Department of Transportation, Public domain