An OPP crash reconstructionist looking into the fatal Highway 17 collision that killed Bryan Casey was questioned in court Tuesday over how he arrived at his conclusion.

Const. Jeff Hewitt was back on the stand Tuesday in the trial of Dr. Christy Natsis, the Pembroke dentist whose SUV collided with Casey's truck on the night of March 31, 2011.

Natsis has pleaded not guilty to impaired driving causing death, dangerous driving causing death, and exceeding the blood-alcohol limit.

Hewitt became involved in the case in the fall of 2012 when he was asked to submit a report on the crash.

Christy Natsis

Dr. Christy Natsis's trial has lasted close to a year.

He told the court he concluded the collision occurred in the eastbound lane — the direction Casey's truck was travelling — based on the final rest of both vehicles and the gouge and scrape marks on the road.

Hewitt said the tires on both vehicles were the size recommended by manufacturers. Because the tires were the correct size, Hewitt says speed data recorded by the vehicles' control modules is accurate.

The court had earlier heard from a Ford automotive engineer who analyzed information from the power control module of Natsis's Ford Expedition that her vehicle was travelling at about 88 km/h at the moment of impact.

Numbers questioned by defence lawyers

Hewitt told the court he used field notes as well as pictures of the crash to form an opinion on where the collision took place.

But when Hewitt went through the numbers he used to calculate how far the Dodge truck swerved to the right, the defence objected, saying Hewitt was using values not found in his report.

The crown asked Hewitt to recalculate the swerve of the Dodge using the original numbers. Hewitt said officers are often asked to use conservative values to err of the side of caution.

The defence says it plans to revisit Hewitt's calculations during cross-examination.

Hewitt said while he consulted field notes before forming an opinion on where the crash occurred, he denied he incorporated the opinions of fellow crash investigators into his report.