The technology behind smart motorway systems did not work for 21 hours during September the BBC can reveal.
Cameras and radar alert a control room to vehicles which have stopped in live lanes with overhead signs then being used to tell drivers about issues.
However, new data shows the system was not functioning for 3% of operational hours in September, with an outage in October lasting nearly 4 hours.
National Highways said an independent investigation was under way.
At the end of 2020 there were 369 miles of smart motorways in England, including 168 miles without a hard shoulder.
The controlled roads are on the M1 in Yorkshire, the East Midlands, and Hertfordshire, along with parts of the M25 in London. There are also stretches of smart motorway in the West Midlands and North West.
The technology is designed to allow lanes to be closed using the signs upstream of incidents, such as live-lane breakdowns.
Computer failures have meant the software to control the signs has not worked properly.
One failure in October lasted for hours, prompting an apology from National Highways.
Claire Mercer, from Rotherham, has campaigned against the use of smart motorways since her husband's death on a stretch of motorway near Sheffield in 2019.
She said she had repeatedly received messages from staff working for National Highways outlining failures in the system.
"It was terrifying. I was sat here getting email after email saying 'we haven't got any cameras. we can't see what we're doing, it's still not working' and it's just sickening.
"You're just sat here expecting to hear about some terrible catastrophe and there's absolutely nothing you can do."
Sarah Champion, Labour MP for Rotherham Central and Mrs Mercer's MP, said drivers were playing "roulette" every time they drove on the smart sections of motorways.
"We believed National Highways when they said the IT would be there to save us when they took away the hard shoulder," she said.
"The IT isn't working. We have data that proves for between 2% and 10% of the time the equipment that is meant to alert us to disaster doesn't work."
"And that when you think about the number of journeys that we're making up and down on the M1 on our stretch is really, really chilling."
National Highways told the BBC the system was working as intended for 97% of the time in September.
Duncan Smith, Executive Director for Operations at National Highways, said safety remained their number one priority.
"We have robust and well-rehearsed mitigation measures in place to deal with any operational challenges facing our network, including those related to technology and staffing."
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