Lady Susan Zetland, of Aske Hall, near Richmond in North Yorkshire, is accused of dangerous driving after she tried to turn right at the Hargill junction of the A66.
The jury at Teesside Crown Court heard there were signs saying the turning was closed and cones preventing it.
Appearing in the dock in bright pink trousers and a floral top, the 77-year-old denies the charge, claiming her driving had been careless but not dangerous.
She was accompanied by her husband, the 4th Marquess of Zetland, who sat in the public gallery.
Prosecutor Paul Newcombe told the court that on July 4 while on her way to the gym in her Subaru Outback 4x4 she reached the junction at just after 6am.
She then almost hit a lorry by trying to pull out but immediately tried again, causing a Nissan Qashqai to smash into the side of her car. This spun her on to the other side of the carriageway, where she was hit by a truck.
He said: “She admits the crash is her fault but she says her driving was careless driving whereas we say it was dangerous driving.
“She was driving her Subaru Outback and was on her way to the gym. She approached the A66 by the Hargill junction and there were some roadworks and some restrictions on the road.
“She ignored the cones and the signs and did not keep a proper look out for traffic and the crash was caused.”
The jury was shown dashcam footage from the Qashqai, which showed her £30,000 Subaru narrowly missing the truck before pulling out into the road.
“It’s not just a case of careless driving but driving that falls far below that of a competent driver,” said Mr Newcombe.
He said: “The road conditions were good, she was not hampered by night-time driving. She was not hampered by the weather, there was no rain and no fog. Visibility was clear.”
The case continues.