Police allege a truck driver involved in a terrifying concrete crash was on his phone and not wearing a seatbelt when two massive slabs broke free and slid into parked cars on a busy motorway.

Extraordinary video shows the two 15‑tonne pieces tearing off the back of his heavy transport truck on July 10 at Bolivar on the North–South motorway in Adelaide’s north, skidding towards three vehicles and narrowly missing two people.

WATCH ABOVE: Adelaide truck driver reported over concrete crash on busy motorway

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The load was headed to the Torrens to Darlington worksites — a multimillion-dollar state-funded road infrastructure project — when the Para Hills West truck driver slammed on his brakes after a ute merged in front of him.

Miraculously, two people escaped serious injury after being struck by their cars as they waited on the motorway shoulder due to a mechanical issue.

One person did break their arm in the crash.

A car merged in front of the truck moments before the concrete came off the truck Credit: SAPOL

The transport of concrete was temporarily banned following the incident, while an investigation was carried out. The Department For Infrastructure and Transport has now capped loads of concrete with only 20 to be transported to the worksite daily.

“These new processes we’ve put in place have been thoroughly reviewed and checked in conjunction with SafeWork and Comcare. We’re confident the transport of these segments will be done safely for the rest of the operation.,” DIT CEO Jon Whelan told 7NEWS.

Police also confirmed the truck driver has lost his licence for 12 months.

The 38‑year‑old has been reported for causing harm by dangerous driving, using a mobile phone while driving, failing to wear a seatbelt and a severe risk load restraint breach.

He will appear in court at a later date.