An 86-year-old woman admitted driving the wrong way down the A1 in Northumberland at a court hearing.
Driver Joan Gordon, from Dunmow, Essex, panicked as cars swerved to get out of her way as she headed south on the north-bound carriageway near Alnwick.
She admitted careless driving in the June 2005 incident when she appeared at Newcastle Crown Court on Thursday.
She was given a £225 fine, ordered
to pay £100 costs and had eight penalty points put on her licence.
The court heard Gordon had an unblemished licence for 50 years. She is to be sentenced later.
She had denied dangerous driving and the prosecution accepted her guilty plea to the lesser charge, minutes before a jury was due to be sworn in.
A delay in sentencing was caused because of a dispute over how far she travelled along the dual carriageway before she realised her potentially fatal mistake.
The prosecution claims she drove half a mile south from the Alnwick turn-off, while Gordon argues she very quickly realised her error and pulled over immediately.
Judge John Milford said: "The defendant is a lady of impeccable character and has held an unblemished licence for over 50 years."