A TRUCK involved in a smash that killed an elderly couple was allegedly travelling at nearly 40 km/h over the speed limit before its trailer rolled and collided with their car, a Melbourne court has heard.

The trailer, laden with sand, had swerved onto the wrong side of the road, narrowly missing another car as it travelled down a hill and around a sweeping bend, prosecutors allege.

A short time later, the trailer of the truck, which at one point allegedly reached 99 km/h in a 60 km/h zone, rolled over and collided with the front of a Holden Commodore driven by Terence and Moira Codling.

The trailer crushed the car and the couple, both aged 70, died at the scene on Coimadai Road, at Toolern Vale on October 24, 2007.

Truck driver Michael Matthew Aparo, 24, of Glenroy, is facing trial in the Victorian County Court charged with two counts of culpable driving.

After the crash, Aparo allegedly told a bystander he "missed a gear and it got away from me".

He told police he believed it was an 80 km/h zone.

The zone changed from 80 km/h to 60 km/h 880 metres before the accident scene, prosecutor Richard Pirrie told the jury.

Police tests showed the truck was travelling at an estimated 99 km/h 10 seconds before the trailer rolled over, he said.

The truck was travelling at 90 km/h five seconds before the roll-over, the tests allegedly show.

Aparo told police he had driven heavy vehicles since he was 15 and had driven Coimadai Road more than 100 times.

Aparo also faces two counts of reckless conduct endangering life relating to two other motorists.

One of those, Bianca Stewart, "in effect closed her eyes and braked" as she saw the trailer on its side in front of her, the court heard.

Her car hit the trailer and she sustained minor injuries.

In his opening address, Aparo's lawyer Matt Fisher urged jurors to cast aside any bias they may have against truck drivers and look carefully at the evidence.

He said there was no dispute his client was driving the truck and the case was about the circumstances leading up to the accident and why it happened.

"This is a tragic accident and the only proper verdict is one of not guilty to all counts," he said.

The trial before Judge Stuart Campbell continues tomorrow.