Don't Mess with Texas is a slogan for a campaign aimed at reducing littering on Texas roadways by the Texas Department of Transportation. 

 

The phrase "Don't Mess with Texas" is prominently shown on road signs on major highways, television, radio and in print advertisements. For over thirty years, the Don’t mess with Texas campaign has taught Texans the real cost of littering.

Source: dontmesswithtexas.org, wikipedia.org