An inquest in Aberdare has heard that water on the A465 Heads of the Valleys was the main cause of a fatal road crash two years ago.
Rhiannon Davies, 18, and Jade Emanuel, 17, died when their car span out of control during heavy rain.
The inquest had previously heard that ditches which should have drained water away from the road were blocked. The coroner recorded a narrative verdict.
South Wales Trunk Road Agency expressed sympathy to the two families.
The two friends, Rhiannon, from Treherbert, and Jade, from the neighbouring village of Blaencwm, were travelling in Rhiannon's blue MG Rover ZR car along the Heads of the Valleys road between Hirwaun and Merthyr Tydfil.
It was raining heavily and the the inquest heard that the windscreen wipers on other cars were struggling to clear water.
As the two women were travelling up the hill, they pulled out into the central lane to overtake cars.
Witnesses told the inquest they saw the car move one way then the other before turning and crashing into the oncoming traffic.
Both young women, who were not wearing seatbelts, were thrown from the car and died at the scene.
The inquest also heard evidence that one of their front tyres was badly worn.
Those that used the road regularly said that they slowed down in rain because there was always a flow of water down the road on that section of the A465.
Daniel Moore, who had been driving there close to the time of the accident, said his car had "twitched to the right" and he ended up in another lane.
At the top of the embankment is a ditch and a drainage system which should catch the water running off the fields and mountains above.
The inquest heard that ditch had become silted up and shallow, with rocks and grass in one part of the drain.
Under cross examination, David Chan, the area engineer responsible for that section of road, admitted it was likely that this meant it never had been inspected.
Robert Twigg, who patrolled that section of road for the South Wales Trunk Road Agency, said the first time he noticed water on the eastbound road was when he attended a similar but non-fatal accident two months earlier.
He told the inquest he had "used his shovel" to clear debris from the drain entrance and suggested the ditch should be made deeper.
He said it was the first time had been aware the ditch existed.
The inquest has heard that since 2008 work has been done to the ditch and drains and water is no longer a problem there.
The coroner said he will write to the South Wales Trunk Road Agency on the issue of communication which appeared to be lacking between those on the ground and those in charge, and to encourage the regular monitoring of this area in adverse weather conditions.