Transport Secretary James Macharia has announced Wednesday and Saturday as car-free days in Nairobi's pilot programme.
From February 1, the central business district will be closed to personal vehicles.
Selected parking lots will be transformed into open-air markets where traders will be allowed to sell their wares.
The pilot will cover Harambee and Moi Avenues and City Hall Way, the CS said without stating how long the pilot will last.
He said they negotiated the deal with the county government.
The Cabinet Secretary spoke on Monday during a briefing on the progress of the implementation of the Bus Rapid Transport (BRT) system.
Vehicles for the system will arrive any time from February 1, Mr Macharia said.
Sixty four buses from South Africa will be used in the pilot programme, he said, adding that sectoral representatives were concluding negotiations on the acquisition of 11 trains from Spain, for use alongside the buses.
Twenty more trains will be shipped in due course, Mr Macharia said and noted that for efficiency in the city's transport sector, at least 900 BRT buses are required.
"We have given local manufacturers the templates for the buses. After the pilot programme, the private sector can work with them to manufacture the remaining buses," he said.