A Nova Scotia teenager is in critical condition and three others have been injured as a result of a single-vehicle accident Saturday morning on Highway 101.

The 16-year-old girl was airlifted to the Queen Elizabeth II Health Sciences Centre in Halifax after the car she was in rolled onto the shoulder near Exit 3, outside Mount Uniacke, before 9:30 a.m.

"Since that time we've transported a 16-year old female — the most serious out of the four patients," Emergency Health Services spokesman Paul Maynard said. "She was ejected from the vehicle and is suffering from a head injury and was transported by Lifeflight to the QE II."

The three other passengers — a 16-year-old and two 17-year-olds — were taken to hospital with non-life-threatening injuries.

Highway 101 was closed between Exits 3 and 4, a stretch about 30 kilometres northwest of Halifax, for nearly two hours.

The RCMP said a preliminary investigation is underway.

It's not known what caused the accident. Conditions were clear at the time.

The RCMP advised drivers to slow down on highways today. Traffic is heavy during the holiday season.