Cheam is the first area in Sutton to trial a new countdown traffic crossing system.
The lights, at the Junction of Cheam High Street and Cheam Broadway, counts down how long pedestrians have to cross the road after the green man has disappeared.
Transport for London, if successful, will roll out the system across London to address the problem of the 'blackout' between the two lights.
Cheam Ward Councillor Mary Burstow said: "We are delighted that Transport for London has listened to residents and councillors' concerns and fitted this light which will improve safety in the area.
"At the end of the school day, it is particularly helpful for mums with buggies and young children - who will welcome help in crossing at the busy junction."
Independent research commissioned by TfL in 2009 showed that around two thirds of pedestrians do not understand what the black-out period at pedestrian crossing means.
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