A man who witnessed the devastating collision between a tractor trailer and a coach bus testified the impact was so hard that it threw many of the bus occupants onto the roadway.

Martin William Sinneave testified Tuesday he and a friend were driving home from Fort McMurray on May 20, 2005, when they came upon the stalled bus blocking the roadway.

Sinneave said an RCMP officer told him to go around the bus, but he stopped first to go to the bathroom in a ditch.

"I looked over my right shoulder and I witnessed the impact of the semi hitting the bus," he said. ""I remember seeing bodies flying through the air ... It's probably the loudest thing I've ever heard."

Sinneave said the tuck hit the bus so hard that it looked like the coach "bounced" from the impact.

"I knew damn well who was on that bus, those were friends," he said. "I knew they would be hurt but I had no idea what I was getting myself into."

Six workers from the oilsands were killed and another 21 were injured in the crash. The employees were making their way back to Edmonton for some time off.

Inderjit Singh Virk, an Ontario-based employee, was driving the semi tractor-trailer south on Highway 18 north of Gibbons on May 2005 when his truck collided with the bus.

He was charged with six counts of dangerous operation of a motor vehicle causing death and 21 counts of dangerous driving causing bodily harm.

He pleaded not guilty to all counts.

Court documents show an earlier rollover on the highway blocked the road, forcing the bus to turn around.

As the coach-style bus went to change directions, it got stuck -- and it couldn't move out of its position of standing perpendicular to the roadway.

Virk's truck then hit the bus as it stood sideways.

Sinneave also testified the bus was fully illuminated at the time of the crash, saying the bus' headlights, harzard lights and interior lights were all on when Virk's truck slammed into it.

Passengers who stayed on the bus and survived the impact also testified Tuesday, some saying they pushed the bus driver to turn around.

Karl Kentsch said the driver was under "extreme" pressure to turn around.

The trial is scheduled to last 10 days.