Noticing the traffic has been stopped, cross (6,3)

I believe the answer is:

seeing red

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

'has been stopped cross' is the definition.
I can't tell whether this definition defines the answer.

'noticing the traffic' is the wordplay.
'noticing' becomes 'seeing' (I've seen this before).
'the traffic' becomes 'red' (I can't explain this - if you can you should believe this answer much more).
'seeing'+'red'='SEEING RED'

Can you help me to learn more?

(Another definition for seeing red that I've seen is " angry".)

I've seen this clue in the Evening Standard.

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