The falling blast debris landed well outside of the designated danger zone during the incident at Brayford Quarry in Brayford on 24 February 2011.
The rocks narrowly avoided striking a workman who had halted traffic whilst the blasting took place.
Frome-based WCD Sleeman and Sons Ltd, who organised the blast, and quarry operator Hanson Quarry Products Europe Ltd were both prosecuted following the incident.
Barnstaple Magistrates’ Court heard that two cars waiting in the queue on a nearby public road, were hit by flying rock, which dented the bonnet of one and a smashed the windscreen of the other.
HSE inspectors discovered an 8.5kg piece of rock on the other side of the road. Six other smaller pieces of rock were also recovered from the road.
A workman acting as a sentry on the road to manage traffic during the blasting heard the rocks coming through the trees and covered his head with his stop-go board and took cover next to a large van which was waiting on the road.
The driver of the van saw pieces of rock pass over the workman.
WCD Sleeman and Sons Ltd, of Valley View, Vallis Park, Frome, Somerset, was fined £20,000 and ordered to pay £17,000 in costs after pleading guilty to safety breaches.
Quarry operator Hanson Quarry Products Europe Ltd of Castle Hill, Maidenhead, Berkshire, was fined £20,000 with £14,000 costs after pleading guilty to a breach of the Quarries Regulations 1999.
Speaking after the hearing, HSE Inspector of Quarries, Mike Tetley, said: “This was a very serious incident that could easily have led to death or serious injury.
“Blasting operations at quarries are inherently high risk, and these risks must be rigorously controlled by good explosives engineering practice and in accordance with legal requirements.
“Where contractors are involved it is important that appropriate levels of communication and co-operation are in place. It is totally unacceptable for both members of the public and employees to be put at serious risk of being hit by rocks, as happened here in an entirely preventable incident.
“I hope this case sends a clear message to the industry that proper planning and control is required at all times.”