Transport on the busy Maai Mahiu-Narok road was once again disrupted on Monday morning after the section that was cut-off last week dipped again.
The Kenya National Highways Authority (KeNHA) said the road caved in again after it developed a volcanic fault line.
KeNHA corporate communications assistant director Charles Njogu said motorists plying the route had been instructed to use alternative routes, including the Narok-Nakuru road.
"KeNHA has embarked on an immediate emergency exercise to restore the section using rockfill," said Mr Njogu.
"Meanwhile motorists are requested to bear with the situation, which is expected to necessitate traffic interruption, which may last up to four hours."
Mr Njogu said KeNHA might create a traffic diversion should the problem, which has hit commercial vehicles the most, persist.
Maai Mahiu Chief Zachariah Igeria confirmed that the road was impassable and cautioned motorists against using it until it is repaired.
Last Wednesday, motorists plying the route spent the night in the cold after a section of the road was washed away by floods.
The busy road was totally cut off, with a fissure extending more than three kilometres upstream and 200 metres on the opposite end.
Engineers from the China Communications Construction Company immediately embarked on major repairs of the road.
By Thursday afternoon, the road was being used amid fears that it would not last.
Infrastructure Principal Secretary Julius Korir, who inspected the section, said the road had "inherent" weaknesses.
He also hinted at a possible redesign of the road as it is prone to damage during rainy seasons.
KeNHA director-general Peter Mundinia, who was among the high-ranking officials who visited the scene last Wednesday, blamed geology for the damage.
He said the Rift Valley region was prone to volcanicity, adding that repairs would include filling up the fault line.
Former President Kibaki inaugurated the Maai Mahiu-Narok Road on August 26, 2011.
The road is a busy highway that connects Nairobi to the South rift, the world famous Maasai Mara game reserve, western Kenya and Luo Nyanza.
It is also an alternative route to the neighbouring Tanzania through Isibania in Migori County.