Olivia Richardson, then aged 33, from Clapham, was hit by a section of a sign advertising the luxury St George Wharf development as she walked along the pavement at Vauxhall Cross, near Vauxhall Bridge in March 2008.
She was hospitalised for five weeks, including several days in intensive care, and required significant brain surgery.
Formerly a primary school teacher, she is no longer able to work and continues to suffer from multiple permanent effects of her injury.
St George South London Ltd (SGSL), an agent for site owner St George Plc, and A E Tyler Ltd (AETL), then trading as Allsigns, who supplied, erected and carried out cosmetic maintenance of the sign, were both prosecuted by the Health and Safety Executive.
The Old Bailey heard that parts of the timber sign, measuring approximately 12 metres by 3 metres and positioned more than 3m above the pavement, had decayed to the point that it was blown down by a strong gust of wind.
Ms Richardson’s partner recalled hearing a cracking sound as the pair approached traffic lights at Vauxhall Bridge.
He turned to find her lying on her back beneath the sign, bleeding heavily from a deep head wound and slipping in and out of consciousness.
Heavy gusts were recorded on the day of the incident, but such was the poor condition of the sign, expert opinion was that it could have fallen at any time.
The HSE investigation established that the sign had a design life of two years, but had been in position for over nine years and had never once in that time been checked for structural soundness.
St George South London Ltd, of Imperial Wharf, Fulham, was fined £300,000 and ordered to pay £222,692 in costs.
A E Tyler Ltd, of Brentford, Middlesex, was fined £60,000 with £22,855 costs.
After sentencing HSE Inspector Loraine Charles said: “This very large advertising sign was positioned overlooking an extremely busy junction at the south end of Vauxhall Bridge, adjacent to an entrance to Vauxhall underground station and opposite the bus station.
“Tragic as the consequences were for Ms Richardson, they could have been even worse had this incident happened during rush hour and not on a Sunday afternoon.
“Throughout the time that the sign remained in place, St George South London Ltd made no sensible efforts to ensure that the structure was safe. Meanwhile, A E Tyler Ltd continued to carry out cosmetic maintenance to the sign, and to make changes to its appearance for many years after it had come to the end of its safe life, without alerting St George to its obligation to ensure that the sign was properly inspected and maintained.”