Traffic lights have been the subject of a few jokes. This joke was cited in 1929:
 
“What makes the traffic light turn red?”
“Maybe they’re blushing to think how green they’ve just been.”
 
This joke was cited in a 1943 newspaper and is still frequently told:
 
“Why does a traffic light turn red?”
“You’d turn red, too, if you had to change in the middle of the street.”
 
   
6 April 1929, Harlingen (TX)

, “The Bright Side,” pg. 4, col. 2:
Now It Can Be Told
“What makes the traffic light turn red?”
“Maybe they’re blushing to think how green they’ve just been.”
 
Old Fulton NY Post Cards
20 January 1943, Watkins Express (Watkins Glen, NY), “Nail Pullers,” pg. 3, col. 5:
“Why does a traffic light turn red?”
“You’d turn red, too, if you had to change in the middle of the street.”
 
11 October 1943, Morning Advocate (Baton Rouge, LA), pg. 3, col. 7:
“Why does a traffic light turn red?”
“You’d turn red, too, if you changed in the middle of the street.”
 
22 April 1944, The Bee (Danville, VA), “The Beehive,” pg. 4, col. 3:
Junior—Why does a traffic light turn red?
Senior—You’d turn red, too, if you had to change in the middle of the street.
 
6 December 1961, The Post-Standard (Syracuse, NY),  “Fun Time—The Riddle Box,” pg. 25, col. 8:
Why did the traffic light turn red?
Because it had to change in the middle of the street.
 
2 December 1972, Xenia (OH) Daily Gazette, “Mini Jokes,” The Mini Page by Betty Debnam, pg. 3, col. 1:
Q. Why did the traffic light turn red?
A You would too if everybody was watching you change.
Bonnie LeValley-Xenia
 
Google Books
August 1973, Boys’ Life, “Think and Grin,” pg. 66, col. 4:
Klaus: Why did the traffic light turn red?
Gerhardt: Well, if you had to change in front of all those people, you would too! — John Wettermark, Mobile, Ala.
 
Google News Archive
2 February 1978, The Free Lance-Star (Fredericksburg, VA), “Fun Time—The Riddle Box,” pg. 6, cols. 1-2:
Why did the traffic light turn red?
Because it had to change in the middle of the street.
 
Twitter
Anna Teoh
‏@annateoh
Q: why did the traffic light turn red? A: bcos he had to change in front of everyone.
11:34 PM - 3 May 2009
 
Twitter
paul steele
‏@paul_steele
Why did the traffic light turn red? You would to if you had to change in the middle of the street.
3:36 AM - 25 May 2009
 
Google Books
The Mammoth Book of Really Silly Jokes
By Geoff Tibballs
London: Constable & Robinson Ltd.
2011
Pg. 369:
Why did the traffic light turn red?
You would, too, if you had to change in the middle of the street.
 
Daily News (New York, NY)
Ian Beattie is mum on the questionable future of his character Meryn Trant on ‘Game of Thrones’; Raven-Symone has moody hair
BY DON KAPLAN
NEW YORK DAILY NEWS Sunday, June 14, 2015, 2:00 AM
(...)
All this reminds of us an old joke: Why did the traffic light turn red? ... Because it saw the other one changing. Badaboom!