The Kenya Highways Authority on Thursday closed the Maai Mahiu-Narok road indefinitely because of dangerous cracks.
Engineers from the authority will assess the road and decide what will be done.
Cracks, believed to be caused by weak underlying volcanic soil, appeared on sections of the road a fortnight ago following heavy rains.
“We have decided to close the road because of the cracks that run across the road,” said authority spokesperson Claire Ouko.
“We have been looking at the situation and as of today, the cracks were as wide as 25 millimetres and widening.”
Motorists heading to and from the western parts of the country will have to use alternative routes, such as the main Nairobi-Naivasha highway.
The authority is not certain when the road will be reopened.
“It is unlike other natural disasters like the washing away of a bridge, so until the engineers assess the situation we can not say how soon we can reopen the road,” said Ms Ouko.
On May 10, a section of the Maai Mahiu- Narok road was washed away by floods near Suswa market.
The highway authority issued a notice urging motorists on the route to be extra cautious as they approached the area.
Heavy trucks weighing more than 10 tonnes were directed to use the Maai Mahiu-Naivasha route, which has now been closed.
President Kibaki inaugurated the Maai Mahiu-Narok road on August 26, 2011. The road links Nairobi to the famous Maasai Mara Park, as well as to western Kenya.
The closure of the highway is likely to affect tourists travelling to the Maasai Mara and also motorists travelling to Narok and western Kenya.