Some £85,000 was spent by the council to improve a nine mile (14.5km) section of the A4059, between Aberdare and Abercynon, which is prone to flooding.
It was part of a £23m scheme to improve roads in the area.
Council leader Andrew Morgan said road improvements "continue to be a key priority when it comes to allocating our financial resources".
The road, known locally as the "Letty Turner's bends" has also had its drainage system improved.
Councillor Morgan said: "We will be investing over £23m in highways improvements, infrastructure and strategic projects over the next three years - this is on top of the £53m we have already invested since 2011 meaning that by 2018/19 we will have invested over £76m in these key areas in recent years."

The 35-year-old victim was attacked outside Barclays Bank in London Road, North End, Portsmouth at about 15:20 BST on Saturday.
The men, aged 41 and 42, were arrested on suspicion of murder and conspiracy to murder. They are both in police custody.
It is understood the victim's injuries are not thought to be life-threatening.
Police said they are appealing for witnesses.

Christopher Colegate, 69, was found in the road at the junction of Heath Avenue and Darby's Lane in Poole, at about 19:30 GMT on Friday 13 December.
He suffered significant injuries and later died at Poole Hospital.
Despite numerous appeals and rewards of £11,000 being offered, the driver of the car has never been found.
Dorset Police have renewed their appeal for witnesses.
Officers believe a light-coloured VW Golf Mark 4, made between 1998 and 2005, was involved in the collision.
Mr Colegate, from Poole, walked with a "distinctive stoop" and used two walking sticks.
Inspector Matt Butler, of the Dorset Police traffic unit, said: "We still do not know exactly what happened to Mr Colegate.
"We know that a number of cars were in the area at the time of the collision.
"Some we have traced but we have not been able to identify a Golf driver that travelled along Heath Avenue toward Wimborne Road at around the time of the incident."
Dorset Police and Crimestoppers are still offering a £10,000 reward, while Oakdale Conservative Club, which Mr Colegate regularly attended, has offered an extra £1,000.

The bank said that "due to changes in commercial priorities" it would not sponsor the event beyond 2016.
It has sponsored the event, which features the top eight men's players in singles and doubles, since 2009 when it moved to London's 02 Arena.
Barclays has also dropped sponsorship of the Premier League and London's "Boris bikes".
A spokesman said the bank was going through a period of change and it was re-assessing the way its portfolio of sponsorships worked for the company.
The bank has not disclosed the value of the tennis deal.
Separately the Daily Mail reported that the World Tour Finals would remain at the 02 Arena until at least 2018. There had been speculation that it would move elsewhere - with interest from North America and Abu Dhabi.
This year's event takes place from 15-22 November.
Work to help prevent flooding on a main road in Rhondda Cynon Taff has been completed.
[NEXT_CONCEPT]
Two men have been arrested after a woman was stabbed in the back as she walked in a busy shopping street.
[NEXT_CONCEPT]
A hit-and-run driver who fled a collision in which a man was killed in Dorset is still being hunted a year on.
[NEXT_CONCEPT]
Barclays Bank is to end its sponsorship of the ATP World Tour Finals tennis tournament.