Accident reconstruction expert Dr Michael Tay wrapped up his evidence on Wednesday on the "combination of factors" that could have led Indian national Sakthivel Kumaravelu to fall into the path of the bus which ran over him.

Among the reasons suggested by Dr Tay were wet roads from light rains that day. Mr Sakthivel was also taking large steps as he ran to chase after the bus. His high blood alcohol content of 217mg of ethanol per 100ml of blood meaning it would be "harder to maintain balance".

His left hand was also holding an umbrella, preventing him from swinging his hand in a natural movement and thus affecting the stability. This, as his right palm was on the bus, meant that he would have to match his movement and speed equally to that of the bus". Dr Tay said: "If he was just running without physical contact with the bus, he would have been more stable. Instead, he had to follow the curved path (as the bus was "off-tracking" while it made a left turn onto Race Course Road from Tekka Lane). This complicated his movement."

The fatal accident involving Mr Sakthivel, a 33-year-old construction worker, in Little India on Dec 8 erupted into Singapore's worst public order disturbance in over four decades, with 49 Home Team officers injured and 23 vehicles damaged.

According to video footage shown in court on Wednesday, the first day of the Committee of Inquiry (COI) hearing into the riot, Mr Sakthivel had boarded the BT & Tan private bus bound for his dormitory in Jalan Papan when it was almost full.

He was asked to get off, first by an unidentified foreign worker and then by bus timekeeper Wong Geck Woon, after he dropped his bermuda shorts on the bus.

He consented, and alighted without being "pushed or manhandled", said Senior State Counsel David Khoo in his opening statement.

Mr Sakthivel then walked along Tekka Lane towards Race Course Road, chasing after the bus after he saw that it departed. Near the junction, he placed his right palm on the side of the bus, only to trip and fall seconds later.

Mr G. Pannir Selvam, a former Supreme Court judge appointed to chair the four-man COI panel, questioned why the driver, Mr Lee Kim Huat, 55, had failed to notice Mr Sakthivel, given the four cameras mounted on the bus.

Dr Tay said, of the "cognitive workload" of the bus driver, that he would have to navigate human traffic on the congested road, with "20 to 30 pedestrians in the vicinity of the bus". There were also two stationary buses parked along Tekka Lane.

Although there were four cameras on board the bus, Dr Tay said the images on the dashboard were small - measuring 4.3cm by 7.65cm per screen - and that Mr Lee would have to look down rather than on the roads.

"During the critical junction turning, just before the deceased fell, he would not have been able to see the deceased," said Dr Tay, due to his smaller size and profile, and his "lower luminance" compared to the glare of the headlights of surrounding vehicles.