Speed restrictions were imposed and one lane of the northbound carriageway was closed for around two hours following reports of the 400-metre spill yesterday afternoon.
A spokesman said the liquid had been identified as hydraulic oil, which had probably leaked from a lorry as it crossed the bridge.
He said: “Because the bridge was closed for two hours, it will take a while [for built-up traffic] to clear.
“Unfortunately, there will be long delays for people getting home.”