Rescue workers from the Kenya Red Cross arrived at the scene moments later but they are having a hard time because there is a snarl-up.
Mr Peter Njoroge said he had been trailing the truck in his car when it suddenly veered off its course to the lanes of oncoming vehicles. It exploded shortly afterwards.
“I was a distance away and that enabled me to slow down and reverse,” he told the Nation.
“It was a huge explosion and other motorists had little chance to react.”
The highway, the main artery that links the city to western Kenya, and on to neighbouring countries of Uganda, Rwanda, Democratic Republic of Congo and South Sudan.
It is considered by the World Health Organization among the most dangerous roads to drive on. At least an accident happens every three days.