Police officers allegedly smashed a car window as a woman who said she felt “too threatened” to leave sat in the driver’s seat with a baby on her lap.
A complaint has been made to Hertfordshire Police about the episode said to have happened on the A10 in Cheshunt.
The woman, her sister and her niece had been at a sales conference and were heading home to the Manchester area.
The force said it was investigating the complaint.
A passenger, who asked for her name to be withheld, said there was no clear reason why they were pulled over on their approach to the M25 at about 01:45 BST on Thursday.
One of the women in the BMW said they were driving at about 30mph in a 50mph zone.
The woman said they were first asked if they were lost. It later emerged the driver thought she was insured on her sister’s vehicle but was not.