Police, paramedics and fire crews were called to a crash between a car and a van in Kings Langley last night.
Two people were taken to Watford General Hospital by ambulance after the smash in Tower Hill, Chipperfield, at about 6.30pm yesterday.
Firefighters from Watford and Hemel Hempstead worked for an hour to cut free two casualties who were trapped in a white Kia Sportage after the crash, which also involved a white Vauxhall Combo van.
Police traced two more people who had left the scene before officers arrived.
One of them had suffered an injury to his head and was also taken to hospital.
The road was closed off while emergency services dealt with the incident and reopened at around 10.40pm.
A man has been arrested on suspicion of drink driving and investigations are ongoing.
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