Slows down rate travelled on roads (7)
I believe the answer is:
retards
'slows down' is the definition.
('retard' can be a synonym of 'slow down')
'rate travelled on roads' is the wordplay.
'travelled' indicates an anagram.
'on' means one lot of letters goes inside another.
'roads' becomes 'rds' (abbreviation).
'rate' anagrammed gives 'etar'.
'etar' placed inside 'rds' is 'RETARDS'.
(Other definitions for retards that I've seen before include "Slows the growth of" , "decelerates" , "Holds back, slows down." , "Delays, slows the progress of" .)