A woman had to be cut from her car after an accident in Markyate.
Three cars were involved in the incident on the A5 on Sunday.
At around 3.50pm, a grey BMW, white Nissan Qashqai and red Toyota Yaris collided on the road between the Holiday Inn and the junction with London Road.
It is believed the BMW had been driving northbound while the other two vehicles were travelling south.
A female passenger of the Toyota was trapped inside the vehicle as a result of the damage and had to be cut out by fire officers. She was treated by air ambulance doctors before being taken by land ambulance to the Luton and Dunstable Hospital with serious leg and arm injuries.
A man and two young children, who were also travelling in the Toyota, suffered slightly less serious injuries and were taken to the same hospital by a normal ambulance. None of the occupants of the other vehicles required hospital treatment.
The road was closed for a number of hours while the Forensic Collision Investigation Unit examined the scene as part of the investigation.
Anyone with information should contact Hertfordshire Constabulary via the non-emergency number 101, quoting incident reference 462 of February 19.
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