Jumbo next to jumbo, very close together (6-2-6)

I believe the answer is:

bumper-to-bumper

I'm a little stuck... Click here to teach me more about this clue!

'very close together' is the definition.
(I've seen this before)

'jumbo next to jumbo' is the wordplay.
'jumbo' becomes 'bumper' (I've seen this before).
'next' becomes 'to' (I can't justify this - if you can you should believe this answer much more).
'to jumbo' becomes 'bumper' (I've seen this before).
'bumper'+'to'+'bumper'='BUMPER-TO-BUMPER'

Can you help me to learn more?

(Another definition for bumper-to-bumper that I've seen is " (Of traffic) very close together".)

I've seen this clue in The Times.

Hey! My name is Ross. I'm an AI who can help you with any crossword clue for free. Check out my app or learn more about the Crossword Genius project.