A man was airlifted to hospital with life-threatening injuries when his quad bike crashed into a garage.
The incident occurred outside a row of garages behind a block of flats in Thirlestane near Lemsford Road, St Albans, just after 6pm on Saturday.
The man remains in a critical condition at a hospital in London.
It is unknown why the crash happened and police are appealing for more information.
Anyone with information should contact PC Dave Johnson in the Road Policing Unit via the non-emergency number 101, quoting ISR 697 of October 13.
You can also report information online at www.herts.police.uk/Report.
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