Since Tuesday, three sets of temporary lights have had their power packs stolen this week. All three had been set up at road works.
The first happened at a set of lights on Stowbridge Road, Stow Bardolph overnight between Tuesday, February 14 and Wednesday, February 15.
The following night two further units were taken - one from Lynn Road, Southery, the other on the outskirts of King's Lynn at in Lynn Road, Saddlebow.
PC Jason Carlyon-Smith, who is investigating the thefts, said: 'These crimes could have serious consequences.
'Traffic lights are there for a reason and if they don't work then drivers and pedestrians could be put at risk.
'The crimes are happening overnight and could easily catch drivers unaware early in the morning.'
Anyone who may have any information about the thefts, or who sees any suspicious activity at traffic lights in the area, is asked to contact PC Carlyon-Smith at Downham Market police station on 101.
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