Too fast for the road junction ahead (3)

I believe the answer is:

ton

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'too fast for the road' is the definition.
I can't judge whether this defines the answer.

'junction ahead' is the wordplay.
'junction' becomes 't'.
'ahead' becomes 'on' ('on' can be a synonym of 'ahead'**).
't'+'on'='TON'

Can you help me to learn more?

(Other definitions for ton that I've seen before include "100 mph, runs, etc" , "20 cwt" , "Two thousand, two hundred and forty lbs avoirdupois" , "Two thousand, two hundred and forty lb" , "A slang 100mph - heavy" .)

I've seen this clue in the Evening Standard.

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