Roadside observers a Merc passed travelling around Spain (5,7)

I believe the answer is:

speed cameras

'roadside observers' is the definition.

'a merc passed travelling around spain' is the wordplay.
'travelling' indicates an anagram.
'around' means one lot of letters goes inside another.
'spain' becomes 'e' (abbreviation for Espana).
'a'+'merc'+'passed'='amercpassed'
'amercpassed' anagrammed gives 'spedcameras'.
'spedcameras' enclosing 'e' is 'SPEED CAMERAS'.

(Another definition for speed cameras that I've seen is " Roadside snappers".)

I've seen this clue in The Telegraph.

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