“What language does a billboard speak?”/“Sign language.”

A popular children’s joke about a billboard is:
 
Q: What language does a billboard speak?
A: Sign language.
 
The joke has been cited in print since at least 2003.
 
 
Google Groups: rec.humor.jewish
Puns of the Weak 04-04-03
Stan Kegel
(...)
What language does a billboard speak?
Sign language! (Jerry, Age 13 )
 
Google News Archive
29 May 2008 The Day, pg. D1, col. 1:
What language does a billboard speak?
Sign language.
 
Google Books
Laugh-Out-Loud Jokes for Kids
By Rob Elliott
Grand Rapids, MI: Revell
2010
Pg. 16:
Q: What language does a billboard speak?
A: Sign language.
 
Google Books
The Happy Classroom:
From Ha-Ha to A-Ha
By Janet Aaker Smith
Marion, IL: Pieces of Learning
2011
Pg. 95:
What language does a billboard speak?
Sign language.
 
Twitter
Hamilton Relay
‏@HamiltonRelay
Question: What language do billboards speak?
Answer: sign language #TellAnOldJokeDay”
5:14 PM - 24 Jul 2014
 
Twitter
ComedyCity
‏@ComedyCityKC
The ComedyCity Official Groaner Joke of the Day:
What language does a billboards speak?
Sign language!
11:53 AM - 23 Jun 2015