Three more people have succumbed to their injuries following an accident on Sunday afternoon near Kikopey, Gilgil, along the Nakuru-Nairobi highway.
This has raised the death toll to 16, police said.
The three had been injured and rushed to a local hospital, where they succumbed.
Police said 13 were killed on the spot after a head-on collision between a lorry and a 14-seater matatu.
Among those who succumbed to injuries on Monday were two children, police said.
Nakuru County Police Commander Emmanuel Opuru said the victims were in a 14-passenger vehicle headed to Nakuru when it collided head-on with a truck.
"The matatu was overtaking and then it rammed into the trailer causing 13 fatalities and three others seriously injured," he said on Sunday.
The victims had hired the matatu from Nairobi to head for a family event.
A probe has been launched to establish more details on the deadly accident. This has left members of the same family whipped out.
The accident took place hours after six people, including occupants of an ambulance, died a few kilometres away along the same highway.
This increased to 21, the number of those killed in accidents within hours on Sunday, police said.
On Saturday evening, a patient, her husband, two more relatives, a nurse and the ambulance driver all succumbed in the accident that occurred near Kimende along the Nakuru-Nairobi Highway.
Witnesses say the ambulance, affiliated with St Mary's Mission Hospital, Elementaita, was speeding, rushing a critically ill patient to the hospital, when it suddenly lost control and crashed.
The aftermath of the accident was a mangled wreck that claimed the lives of everyone on board instantly.
In a separate incident, an accident happened along the same road at Gilgil, triggering a major traffic snarl-up on the busy highway.
Several vehicles, including a truck, were involved in the crash between Kikopey and St Mary’s, although the number of casualties has yet to be confirmed.
According to eyewitness reports, several passengers were feared to have been injured in the incident.
The truck overturned, completely blocking the highway and bringing movement to a standstill, forcing motorists to wait for police response to unlock the gridlock.
The injured were taken to hospitals.
