Vehicles were brought to a standstill in Darlington, County Durham, on September 9 after traffic lights suffered a breakdown due to a slug shorting a circuit board.
Council engineers who inspected the control box at a busy junction in Whinfield Road discovered the dead slug inside.
Residents had complained about the fault to councillor Chris McEwan, whose ward neighbours the traffic lights.
Mr McEwan said: "Somehow the slug managed to infiltrate the control mechanism, sit on the circuit board and then it got fried.
"We don't know how long the slug had been there.
"Unfortunately, it was dead by the time we found it, so we were unable to question it.
"Sadly, you just can't legislate for a rogue slug trying to take out Darlington's traffic system."
The control box has now been resealed, while the council thanked motorists for being patient.
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