Sean Luke Hale, 30, from Cwmbran, was hit by the vehicle while crossing the carriageway to collect traffic cones from the central reservation during road resurfacing in 2006.
A Health and Safety Executive (HSE) investigation found that his employers, Newport-based R P Traffic Management, had failed to ensure that a safe system of work was in place, including the use of appropriate signs to warn drivers that workmen were on the carriageway at the time.
Cardiff Magistrates Court heard that Hale was working with a colleague at around 9.30pm on 8 September 2006 when the incident happened.
As trainees, both men were being supervised at the time but the quality of supervision was called into question. The court heard both men were observed crossing the carriageway in a dangerous manner on a number of occasions on the same night.
R P Traffic Management pleaded guilty to failing to ensure the safety of two trainee traffic management operatives under Section 2(1) of the Health & Safety at Work Act 1974. It was fined £100,000 and ordered to pay £22,000 costs.
The firm installs and remove signage, cones and other materials to manage the flow and speed of traffic when construction works and other activities take place on roads.
HSE inspector Wayne Williams said: "On the night in question, the gang of three workers consisted of one foreman and two trainees. This was not adequate supervision in what is a high risk working environment.
"Trainee roadworkers should always be under a high level of supervision when undertaking this kind of work as part of an overall safe system of work on the job.
"Everyone involved in work on high speed roads should learn from this tragedy and consider whether they are doing enough to prevent needless deaths and injuries. Motorway works can be very dangerous places unless a high degree of control is maintained.
"Advanced warning signs should always be used when people are working on motorway roadworks to alert drivers that people may be in the road and exercise caution as they approach."