The warning follows an incident in August 2005 where an employee was knocked down and seriously injured while painting road lines in Hammersmith and Fulham, London.
Mr Stephen Jordan, trading as Central Linemarkings of Aldershot, Hants was fined £5000 at the City of London Magistrates Court after pleading guilty to a charge under Section 2(1) of the Health and Safety at Work etc Act 1974. He was also ordered to pay £10,000 costs.
Lisa Chappell, HSE Inspector, said: "This unfortunate accident shows how important it is for road contractors to provide appropriate instructions, training and supervision to safeguard their workers. Regardless of the duration of the work, roads are clearly hazardous places, and workers must understand the measures they should take to protect themselves and warn other road users of their presence."