Devon County Council is reporting problems with the traffic signals at Penn Inn Roundabout, Newton Abbot.
The fault has been caused by a communications cable between the signals being accidently severed by contractors.
The Council says that the signals are working, but will not automatically adjust their timing to optimise traffic flow.
It means that congestion is likely at peak hours.
The fault has been reported to BT, whose engineers will be required to fix the severed cable, and the council currently understands that this will be later in the week.
The Council is apologising for the inconvenience.