A slug brought cars to a standstill when it crawled into a traffic light and shorted the circuit board.
Council engineers were called to a busy junction in Darlington, County Durham, when the lights failed last Friday morning.
They found the dead slug when they opened the control box in Whinfield Road near the town's Asda supermarket.
Councillor Chris McEwan, whose ward neighbours the traffic lights and who has received complaints from residents about the fault, 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 council has since resealed the control box and thanked drivers for their patience.