The A465 between Glynneath and Hirwaun has now reopened, South Wales Police say.
One person was taken to hospital earlier in the evening after 12 vehicles were involved in a collision.
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More significant rain will fall overnight with warnings in place for much of Wales.
It comes after a number of mountains in the Rhondda were hit by landslides caused by heavy and prolonged rain during Storm Dennis.
Construction of a dual carriageway was meant to take less than four years but will go on for more than six years.