“A truck carrying onions has overturned. Motorists are advised to find a hard shoulder to cry on” is a joke that originated in the United Kingdom. Scottish stand-up comedian and actor Ronnie Corbett (1930-2016) said on the television show The Two Ronnies at an uncertain time, as quoted in a 1986 book:
 
“And we’ve just heard that a juggernaut of onions has shed its load all over the M-1. Motorists are advised to find a hard shoulder to cry on.”
 
         
Wikipedia: Ronnie Corbett
Ronald Balfour Corbett, CBE (4 December 1930 – 31 March 2016) was a Scottish stand-up comedian, actor, writer, and broadcaster, best known for his long association with Ronnie Barker in the BBC television comedy sketch show The Two Ronnies. He achieved prominence in David Frost’s 1960s satirical comedy programme The Frost Report (with Barker) and subsequently starred in sitcoms such as No – That’s Me Over Here!, Now Look Here, and Sorry!
     
IMDb (The Internet Movie Database)
The Two Ronnies (1971–1987)
Quotes
Ronnie Corbett: And we’ve just heard that a juggernaut of onions has shed its load all over the M-1. Motorists are advised to find a hard shoulder to cry on.
   
Google Books
Word Games
By Gyles Daubeney Brandreth
New York, NY: Harper & Row
1987, ©1986
Pg. 120:
A lorry carrying onions has shed its load on the motorway. Motorists are advised to find a hard shoulder to cry on.
   
Google Books
The Listener
British Broadcasting Corporation
Volume 123
1990
Pg. 15:
“The police tell us that a wagon has shed its load of onions on the M6 motorway. You should use the hard shoulder to cry on.” Norman Thomas (North West Local Radio).
 
Google Groups: uk.people.support.depression
I while you are thinking about that ...
Pub Landlord
6/9/07
(...)
Talking of tears, here in London this evening a juggernaut overturned on the north circular and shed it’s load of onions, the police have advised motorists to find a hard shoulder to cry on….
 
Twitter
Gene Hunt ⭐ ⭐ ⭐ ⭐ ⭐‏
@GeneHunt
A juggernaut of onions has just shed its load all over the M4. Motorists are advised to find a hard shoulder to cry on.
6:19 AM - 24 Jun 2009
     
4 December 2010, Daily Mail (London, UK), pg. 22:
WE’VE just heard that a juggernaut of onions has shed its load all over the M1. Motorists are advised to find a hard shoulder to cry on.
   
4 December 2015, Daily Mail (London, UK), “Ronnie’s little gems: As he turns 85, a feast of Ronnie Corbett’s funniest (and sometimes corniest) one-liners,” pg. 30:
WE’VE just heard a juggernaut of onions has shed its load all over the M1. Motorists are advised to find a hard shoulder to cry on.
 
Twitter
Barry Jackson 🐝‏
@BarryJackson78
“A juggernaut of onions has shed its load across the M1. Motorists are advised to find a hard shoulder to cry on” - Goodnight Ronnie Corbett
6:29 AM - 31 Mar 2016
 
Twitter
🤣 The Dad Joke Man 😉‏
@DadJokeMan
NEWSFLASH ! A lorry carrying onions has overturned on the M6…
Police are urging motorists to find a hard shoulder to cry on….
5:59 AM - 19 Feb 2018