A city man identified as David Lemayian was seen standing in front of a black Mercedes, registration number KCC 600P.
In a video shared by activist Boniface Mwangi on Monday evening, he stays put, dressed in blue jeans, a striped yellow shirt and a satchel bag.
Lemayian then appears to lecture the occupants of the Mercedes about traffic rules while pointing at other motorists who were waiting for clearance.
The National Transport and Safety Authority web portal indicates that the owner of the vehicle is Kapseret MP Oscar Sudi.
It was not clear whether the MP was in the vehicle at the time of the obstruction.
Lemayian's move elicited mixed reactions, Mwangi saying Kenyans should take after him.
Martin Waigwa said Nairobi, where traffic has been a menace partly due to unruly drivers, needs people like Lemaiyan.
"Those are the kind of citizens who give me hope in this society.... I salute you for the sacrifice you made to make sure the law was followed," he said.
"No matter how small the sacrifice ... No matter how many people discouraged you and insulted you ... I think you are a great citizen worth emulating."
Winnie Gathura admired Lemaiyan's courage and asked Kenyan drivers to stop rushing and assuming every one wastes their time.
"We should all do the same to such idiot drivers," he said.
But others said Lemaiyan would have earned himself more than he bargained for, had he encountered an unruly driver.
"Come try that with Rongai matatus," Roggy Kemoi said.
Emmanuel Kebenei scolded the man for acting without knowing why the driver wanted to overlap. He said Lemaiyan should have called the police.
Baba Nelly wanted to know what happened next or if that was just an opportunity for good photographs to be taken.
Martin Njihia told Lemaiyan: "Just so you know, that's not illegal as per traffic laws. It only becomes illegal when he causes an obstruction for oncoming traffic."
One Bsc Davido added: "Overtaking is allowed by the law."
In 2016, traffic police officers had been instructed to arrest and charge government officials found flouting traffic rules.
This followed complaints by members of the public about the driving of their vehicles.
Officers at the police headquarters said they had received complaints about government-owned vehicles overlapping and causing accidents.