Cecil Grant was repairing CCTV cameras used to monitor one lane of the motorway near Clevedon during the night of 24 January 2006, when he fell off a wall into bushes below.
He was taken to hospital and died of his injuries ten days later.
The Health and Safety Executive prosecuted the 42-year Bristol man’s employer Serco and principal contractor Birse Civils for failing to protect Grant when he was working at the site, near Junction 20.
Serco pleaded guilty to breaching two safety regulations and was fined £200,000 and ordered to pay costs of £36,186
Birse Civils pleaded guilty to one safety regulation breach and was fined £100,000, with costs of £180,093
Bristol Crown Court heard Serco did not plan, manage and monitor the work properly, meaning Grant was not warned about the drop.
Both companies had also failed to cooperate with each other to make sure workers at the site were not put in danger.
HSE inspector Steve Frain said: “There were major failings in the way this work was planned which sadly resulted in Mr Grant’s tragic death.
“When employees are working at height, proper plans must be put in place and workers made aware of possible risks.
“In this case, Mr Grant had not been made aware of the dangerous drop where he was working. If he had been, a tragic incident could easily have been avoided.”