Two people had to free themselves from a car after it crashed on the Heads of the Valleys road in Neath Port Talbot.
The A465 shut for an hour after a car suffered "significant damage" when it hit the hard shoulder crash barrier between Glynneath and Resolven.
South Wales Police said an oil spill covered both carriageways.
South Wales Fire and Rescue Service added a man and a woman freed themselves from their car. The road reopened just after midday.